Coal tar, a tumour initiator, and dithranol, a tumour promoter, are us
ed in the treatment of psoriasis. Topical treatment of mice with pharm
aceutical formulations of these two agents, at therapeutic doses, indu
ced skin papillomas in a classical two-stage carcinogenesis protocol,
while treatment with either agent alone did not. This finding has impl
ications for the use of both agents in combination in the treatment of
psoriasis.