The aims of the present study were to evaluate T-cell-mediated immune response and acute phase response in cats with infectious peritonitis (FIP). In the study, 20 cats with FIP and 10 clinically healthy cats were used. Cats with FIP were divided into two groups as dry form (n=10) and wet form (n=10) based on clinical, radiographic and necropsy findings. Adenosine deaminase (ADA-1) and C reactive protein (CRP) levels were determined by using cat-specific ELISA kits. Serum concentrations of total protein (TP) and albümin (A) were measured by photometric methods. In the study, both serum and peritoneal effusion concentrations of ADA-1 (p<0.001) and CRP (p<0.001) were significantly higher in cats with FIP than control cats. A high positive correlation was obtained between serum concentrations of ADA-1 and CRP in cats with FIP (r=0.62, p<0.001). Significant increases in TP (p<0.01) and globulin (p<0.01) levels and decreases in A (p<0.01) values and A/G ratio (p<0.01) were obtained in cats with FIP. Serum TP (p<0.05) and G (p<0.05) levels were significantly higher, whereas ADA-1 activitiy (p<0.01) was lower in cats with dry form than in cats with wet form of FIP. As a conclusion, both serum and peritoneal effusion samples can be used to determine ADA-1 and CRP, and they can be useful biomarkers for evaluating T-cell-mediated immune responses, inflammation and possibly organ damages in cats with FIP.