The effector arm of antibody-dependent cellular cytotoxicity (ADCC) was evaluated using 51Cr-labelled chicken erythrocytes as targets in BALB/c mice transplanted with the Moloney sarcoma virus-induced tumours T-MSV and MS2, and in C57BL/6 mice transplanted with the chemically induced FS6 sarcoma, Lewis lung carcinoma and B16 melanoma. Tumour-bearing animals showed higher levels of ADCC than normal mice, a stimulation confirmed in MS2-bearing mice, using SL2 lymphoma cells as targets in a cytostasis assay. ADCC effector-cell capacity was higher in animals transplanted with the immunogenic, spontaneously regressing T-MSV than in mice bearing the poorly immunogenic metastasizing MS2 sarcoma. The increased ADCC activity detectable in the spleen of tumour-bearing hosts was not abolished by removal of phagocytic-adherent cells.