Objective: In a previous study we showed that red cell deformability was decreased in cats with non-insulin dependent diabetes mellitus (NIDDM) and it was associated with abnormalities in various theological parameters (blood and plasma viscosity, erythrocyte deformability, hemotological parameters) as well as alterations in red cell membrane proteins. On the other hand, there is limited data concerning the contribution of lymphocyte membrane protein abnormalities and increased lymphocyte rigidity to impaired blood rheology in diabetes. In the present study, we aimed to investigate lymphocyte deformability and alterations in lymphocyte membrane proteins in cats with NIDDM. Material and Methods: In this regard, we analyzed lymphocyte deformability in 10 cats with non-insulin dependent diabetes mellitus (male/female: 5/5, mean age: 3.08 +/- 1.39 years, mean weight: 3.26 +/- 1.25 kg.) and 10 healthy controls (male/female: 6/4, mean age 5.7 +/- 1.91 years, mean weight: 4.56 +/- 1.16 kg.) by using the microfilter technique. We also assessed membrane protein content by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In addition, blood levels of glucose, total cholesterol, highdensity lipoprotein (HDL), low-density lipoprotein (LDL), very low-density lipoprotein (VLDL) and trygliceride were determined. Results: We found that lymphocyte rigidity (-/+) was significantly increased in cats with NIDDM compared to controls (1.215 +/- 0.532 vs. 0.265 +/- 0.42; p<0.01). Blood (3.8 +/- 0.1 vs 4.4 +/- 0.2; p< 0.001) and plasma viscosity (1.2 +/- 0.1 vs. 1.4 +/- 0.1; p< 0.01) were significantly higher in the diabetic group. Blood levels of glucose (99.8 +/- 21 mg/dL vs. 177.9 +/- 53.2 mg/dL; p< 0.001), total cholesterol (106.4 +/- 15.6 mg/dL vs. 183.4 +/- 57.8 mg/dL; p< 0.001), LDL (12.1 +/- 5.3 mg/dL vs. 24.7 +/- 14.6 mg/dL; p< 0.02) and HDL (78.7 +/- 9.5 mg/dL vs. 133.4 +/- 44.9 mg/dL; p< 0.001) were significantly higher in the diabetic group. SDS-PAGE revealed that the band which corresponds to the protein with a weight of 37 kDA had disappeared in cats with NIDDM. Conclusion: We suggest that the observed abnormalities in membrane proteins may play a role in reduced lymphocyte deformability associated with diabetes mellitus and may have a role in increased blood viscosity.