We have examined the ability of interleukin-4 (IL-4) to inhibit the growth of the B.16 melanoma in C57BL/6 mice. Tumours were allowed to become established and then treated locally with 50 ng/day IL-4. Treatment was continued for 7 days and then the tumours were removed and weighed. The results showed that recombinant murine IL-4 (rmIL-4) strongly inhibited tumour growth, with some complete resolutions. In addition, the effect of IL-4 treatment was seen to be systemic; in mice bearing two tumours (neck and thigh), which had been innoculated simultaneously, treatment at the neck site resulted in reduced tumour growth at both sites. This effect was accompanied by an increase in the proportion of lymphocytes in the spleen, particularly T-cells, although tumours at neither site showed much evidence of leucocytic infiltration. These results suggest that local treatment with IL-4 is able to profoundly reduce the rate of tumour growth, even of nodules not receiving treatment directly.