In the present study, we analysed the phenomenon of leukophagocytosis by tumour cells with respect to the age of the animal. Splenic leukocytes from C3H mice at different ages were depleted of macrophages and used as targets against tumour effector cells. Tumour cell lines P388D1, a macrophage lymphoma of mouse origin and LAZ-559, a B cell lymphoma of human origin, were cultured with leukocytes at 37 degrees C and 5 per cent CO2 for 4 h and the percentage of phagocytic tumour cells were examined in cytospin preparations. The results demonstrated that the phagocytic activity of P388D1 cells was 10, 6, 3 and 28 per cent against 1, 2, 6, and 35 week-old mice respectively; while LAZ-559 showed lower activity; 5, 3, 1.5 and 17 per cent against leukocytes of corresponding ages. Susceptibility of old leukocytes to phagocytosis could be detected as early as 30 min after culture with tumour cells, and increased with increasing incubation period. At 4 h, phagocytosis by tumour cells resulted in 38 and 29 per cent reduction in the leukocyte number postculture with P388D1 and LAZ-559 cells respectively. We postulate that an age-related decline in immune function is associated with increased level of phagocytic tumour cells, and that the loss of leukocytes may have adverse effects on the immune system.