Benzoyl peroxide (BzPo), a free radical generator with tumor promoting activity on mouse skin, is shown to promote neoplastic transformation of JB6 mouse epidermal cells in vitro. Repeated exposures to BzPo are required to readily detect promotion of transformation of JB6 cells. Markedly reduced net synthesis of the major epidermal ganglioside, trisialoganglioside GT1b, (GT) occurs with BzPo treatment as with other tumor promoters active in this system. Addition of ganglioside GT prevents transformation by BzPo while retinoic acid does not.