A substance immunologically cross-reactive with insulin (SICRI) was isolated and purified from murine melanoma B16. Biochemical and biological data provide evidence that SICRI and insulin are two distinct biologically active agents. In vitro, SICRI has stimulated the DNA and protein synthesis, cell growth or colony-forming ability of 19 normal and transformed cell lines of human and rodent origin. It indicates that SICRI is a potent nonspecific growth factor. The most pronounced stimulatory effect of SICRI on proliferative capacity of cells is observed with cells at G0/G1 point of the cell cycle.