The antitumour efficacy of batracylin was investigated in vivo against murine tumours. The drug displayed an original spectrum of activity. It was totally inactive against L1210 leukaemia and B16 melanoma, while it was marginally active against ascitic P388 leukaemia. However, the tumour growth of the subcutaneously (s.c.) implanted colon 38 adenocarcinoma (Co 38) was completely inhibited in 80-90% of the mice. Therapeutic efficacy was retained upon oral administration and the drug was able to induce tumour regression in the advanced Co 38 disease. These data justify the selection of batracylin for toxicological studies and possible clinical investigations.