Both melphalan (L-PAM: L phenylalanine mustard) and 5-fluorouracil (5-FU) expressed a cytotoxic effect in vitro on MOPC-315 plasmacytoma tumour cells. However, they differed in their chemotherapeutic effectiveness in BALB/c mice bearing large MOPC-315 plasmacytoma tumours. Therapy with L-PAM, 7.5 mg/kg induced permanent regression of tumours, whereas regression induced by 5-FU, 200 mg/kg, was only transient and the mice dies finally with tumours. Moreover, spleen cells of tumour bearing mice treated with L-PAM exhibited high specific cytotoxic potential in vitro whereas spleen cells from tumour bearing 5-FU treated mice were devoid of cytotoxic potential. Effectiveness of chemotherapy with L-PAM was antagonized by treatment in combination with 5-FU. L-PAM, but not 5-FU potentiated cell mediated contact sensitivity response in vivo and impaired induction of T-suppressor cells by ConA. The parameters mentioned above indicate that L-PAM behaves as an "immunopromoting" drug and 5-FU as a "nonimmunopromoting" drug.