The immunomodulating effect of a new heterologous antigen (PIND-AVI) on helper (inducer) and suppressor T cells was investigated. Primarily decreased T4 values (less than or equal to mean T4 - 2 S.D.) increased after treatment with PIND (P less than 0.005, using Student's t-test; n = 15). Primarily increased T 8 values (greater than or equal to mean T8 + 2 S.D.; obtained for normal persons) decreased after therapy (P less than 0.001; n = 10). An increase of T4/T8 ratio (P less than 0.005 and less than 0.001) to normal values was observed in both cases. T4 and T8 values within the mean +/- 2 S.D. were not influenced significantly (n = 13). In spite of different primary values, the amount of lymphocytes/mm3 was not significantly influenced. It appears advisable to check the results of treatment continuously every few weeks.