The levels of T, B and null lymphocytes in the peripheral blood, draining lymph nodes, and tumour masses at different growth stages in dogs transplanted with canine transmissible venereal sarcoma (CTVS) were determined by immunofluorescence techniques. The tumours were classified at excision into "progressor", "steady state", and "regressor" stages of growth. The percentage of B cells in the lymphocytes infiltrating into the progressively growing tumours (n = 10, 37.3 +/- 7.4%) was significantly higher (P less than 0.025) than that in regressing tumours (n = 21, 26.1 +/- 1.9%). In contrast, the percentage of T cells in the lymphocytes infiltrating into the regressing tumours (n = 21, 61.2 +/- 2.6%) was significantly higher (P less than 0.005) than that in the progressively growing tumours (n = 10, 34.0 +/- 5.1%). The tumours at the steady-state growth stage (n = 9) had 50.8 +/- 5.7% infiltrating T-cells, which was significantly higher (P less than 0.005) than the progressors and lower (P less than 0.005) than the regressors. The percentage of null cells of progressors (n = 10, 26.0 +/- 6.9%) was significantly (P less than 0.025) higher than in regressors (n = 21, 13.5 +/- 2.9%). The draining lymph nodes of progressor dogs (n = 5) had significantly fewer (P less than 0.025) B cells (8.2 +/- 2.3%) than in normal (n = 5, 16.1 +/- 3.1%), regressors (n = 12, 19.1 +/- 1.7%) and steady-state dogs (n = 5, 15.8 +/- 2.6%). Although there was slight lymphopenia and fluctuation of null cells, no significant differences in T- and B-lymphocyte levels were noted in the peripheral blood of the tumour dogs (n = 44) studied.