Peanut agglutinin (PNA) and surface immunoglobulin (SIg) were investigated as markers for T and B lymphocytes in blood and lymphoid tissues of dogs of various ages. In the blood study, 4 age groups (n = 8 dogs/group) were used. The mean (+/- SD) percentages of PNA-positive (PNA +) cells were 68.4 +/- 8.6% (group 1, < 1 year old), 70.3 +/- 9.2% (group 2, 1 to 2 years old), 72.0 +/- 3.7% (group 3, 5 to 6 years old), and 63.8 +/- 10.1% (group 4, 10 to 11 years old). The mean percentages of SIg-positive (SIg+) cells in blood were 32.1 +/- 10.6% (group 1), 43.2 +/- 7.0% (group 2), 34.3 +/- 4.8% (group 3), and 35.0 +/- 6.8% (group 4). The mean total percentages of PNA+ and SIg+ cells were 100 +/-6% (group 1), 113.5 +/- 4.9% (group 2), 106.3 +/- 5.3% (group 3), and 98.9 +/- 9.2% (group 4). The proportions of PNA+ and SIg+ cells in dogs of group 2 were significantly (P < 0.05) different from those in dogs of the other groups. Serial changes in PNA+ and SIg+ cells were investigated in blood of 6- to 29-week-old pups (n = 8). A significant (P < 0.05) transient decrease in PNA+ cells and a corresponding increase in SIg+ cells was observed in pups between 14 and 17 weeks old. Lymphoid tissue specimens and blood samples were obtained from 2- to 6-month-old dogs (n = 11) and from 6- to 12-month-old dogs (n = 10). Percentages reflected the combined data from both groups because there were no significant differences between the 2 age groups. The mean percentages of PNA+ cells were: blood, 68.4 +/- 8.6%; thymus, 86.6 +/- 16.3%; spleen, 29.5 +/-16.0%; lymph node, 48.5 +/- 16.0%; and bone marrow, 30.8 +/- 26.4%. The mean percentage of SIg+ cells were: blood, 32.1 +/- 10.6%; thymus, 3.1 +/- 5.5%; spleen, 69.3 +/-10.3%; lymph node, 55.4 +/- 15.2%; and bone marrow, 65.4 +/- 22.4%. The procedure to identify T lymphocytes in blood and lymphoid tissue was easy to perform, was reproducible, and could be performed on as few as 10(6) cells.