Cats with body cavity effusions are commonly presented to veterinary practices. They might suffer from a variety of conditions, including heart disease, neoplasia, septic process, or feline infectious peritonitis (FIP). Timely diagnosis is essential to institute appropriate treatment. This thesis was designed with the aim to assess the utility of acute phase proteins (APPs) (measured in serum and effusion) to differentiate FIP from other diseases causing body cavity effusions, and to evaluate the ability of different classification schemes and APPs (measured in serum and effusion) to differentiate between exudative and transudative effusions. Eighty-eight cats with body cavity effusions were included in the assessment of utility of APPs in diagnosing FIP, and 65 cats were included in the study evaluating the ability of different classification schemes and APPs to differentiate between exudative and transudative effusions. Of the 88 cats, 44 had pleural effusion, 37 had ascites, five had both ascites and pleural effusion, and two had pericardial effusion. Of the 88 cats, 24 had a tumour, 22 had a cardiac disease, 20 had FIP and 22 were diagnosed with other diseases (mainly septic conditions). Analysis of effusion included total nucleated cell count (TNCC), total protein (TP), albumin, activity of lactate dehydrogenase in effusion (LDHef), and cytological examination. Simplified Light’s criteria (LDHef, effusion/serum TP [TPr]) and serum-effusion albumin gradient (ALBg) were available in 65 cats. Acute phase proteins serum amyloid A (SAA), ?1-acid glycoprotein (AGP) and haptoglobin (HP) were measured using previously validated assays in serum of 88 cats and effusion of 67/88 cats. Statistical analysis was performed using non-parametric tests. Diagnostic utility of parameters measured in serum and effusion was assessed by calculation of sensitivity, specificity, and accuracy, and by receiver operating characteristic analysis. Level of significance was set at p<0.05. The study revealed that concentrations of all three assessed APPs differed between cats with FIP and other diseases in both serum and effusion, and that AGP measured in effusion was the best marker to differentiate between FIP and other diseases, with an area under the curve (AUC) of 0.95, and a cut-off value of 1550 ?g/ml having 93% sensitivity and 93% specificity for diagnosing FIP. The study also confirmed superior diagnostic accuracy of simplified Light’s criteria, LDHef alone, TPr alone, and ALBg in differentiating between exudates and transudates in comparison to the traditional veterinary scheme, based on TNCC and TP. In addition to previous smaller studies that applied Light’s criteria to pleural effusions only, this study has proven their applicability to ascites as well. This investigation also found that APPs measured in serum and effusion (apart from SAA measured in effusion) were able to differentiate between exudates and transudates categorised according to aetiological classification but not using the traditional veterinary scheme. This conformity of APPs with aetiological classification but not the traditional veterinary scheme can be interpreted as another piece of evidence in favour of the aetiological classification. Alpha-1-acid glycoprotein measured in effusion was the best acute phase marker to differentiate between exudates and transudates, with an AUC of 0.79, and a cut-off value of 340 ?g/ml having sensitivity of 87% and specificity of 61% in identifying an exudate. This research therefore provides further evidence of usefulness of measurement of APPs in veterinary medicine.