Prostaglandins of the A series (PGAs) have been previously shown to inhibit the growth and to stimulate the differentiation of Friend erythroleukaemic cells (FLC) in vitro. In the present report we analysed the effect of PGA treatment in vitro on FLC tumorigenicity, and in vivo on FLC proliferation and on natural killer (NK) activity. PGA1 pretreatment of FLC in vitro for 5 days before inoculation into syngeneic mice slightly delayed tumour appearance, but did not significantly alter the pattern of tumour growth or mice survival, indicating that PGA1, at least in the conditions studied, did not affect FLC tumorigenicity. Daily treatment of mice with a long-acting synthetic analogue of PGA2 (16, 16 dimethyl-PGA2-methyl ester, di-M-PGA2) delayed tumour appearance, inhibited tumour growth, as measured by tumour weight and diameter, and increased the median mice survival time by 15-35%, depending on the schedule of treatment. Daily treatment with di-M-PGA2 strongly suppressed NK activity in normal mice but had no significant effect in tumour-bearing immunodepressed mice. PGA treatment of effector or target cells in vitro, or PGA added during the NK assay, had no effect on NK activity. We suggest that the chemotherapeutic effect of PGA is due to a direct action on tumour cell replication rather than to a stimulation of the host NK activity.