The aim of this study was to evaluate peripheral blood lymphocyte subpopulations, neutrophil phagocytic capacity and proteinogram characteristics in mares, during the last trimester of pregnancy and in postpartum. Measurement of phagocytosis and quantification of T-lymphocyte subsets were done by flow cytometry. Quantification of T-lymphocyte subsets was performed with monoclonal antibodies specific for CD2, CD3, CD4 and CD8 cell markers. Natural killer and B-cell counts were estimated mathematically. Serum proteinogram was obtained by electrophoresis. No significant differences were observed between gestation and postpartum on CD4(+), CD8(+) and NK+ lymphocyte subsets, CD4: CD8 ratio and phagocytosis. The percentage of cells expressing CD3 (64.2 +/- 1.8) and CD2 (68.4 +/- 1.7) (Mean +/- SEM) was reduced during gestation vs postpartum (69.7 +/- 1.5 and 73.8 +/- 1.4 respectively) (p < 0.05). During pregnancy, CD19(+) (31.6 +/- 1.7) was higher than in postpartum (26.2 +/- 1.4) (p < 0.05). Total T cells (2911 +/- 227 cells/mu l), T helper cells (2144 +/- 169 cells/mu l) and T-cytotoxic cells (767 +/- 68 cells/mu l) were depressed in pregnancy, when compared with postpartum (4093 +/- 337 cells/mu l; 3004 +/- 276 cells/mu l; 1089 +/- 94 cells/mu l respectively) (p < 0.01). Total white blood cell count was reduced during pregnancy (8815 +/- 427 cells/mu l) with respect to postpartum (10742 +/- 446 cells/mu l) (p < 0.01), while neutrophil count did not change. Total proteins, albumin, alpha 1,alpha 2,beta 1, beta(2), gamma globulins and albumin : globulin did not differ. Our results suggest that the physiological immune depression occurring in mares, during gestation might be due to T-helper and T-cytotoxic lymphocytes reduction.