STUDIES of the mechanism through which MSH acts in mammals have been restricted to intact animals or to melanomas grown in animals1,2. We describe here a mouse melanoma cell line cultivated in monolayer which when exposed to MSH shows large increases in tyrosinase activity and melanin content, as well as changes in growth characteristics and cellular morphology. In addition, we have found that adenosine 3',5'-monophosphate (cyclic AMP) or its analogue N6,O2'-dibutyryl cyclic AMP (diB cyclic AMP) will substitute for MSH, supporting previous evidence that MSH probably acts through cyclic AMP in mammals1,3.