Pigment spread is the natural or experimentally procured (through grafting) progressive encroachment of pigmentation from black or red skin areas into juxtaposed white skin areas, or from black skin areas into red skin areas in spotted guinea pigs and other mammals. So far as spread from black into white skin is concerned, it had previously been shown that migration of epidermal melanocytes into skin lacking homologues of these cells was responsible. However, since red skin already has its own complement of phenotypically "red" melanocytes, the intriguing possibility remained that when black pigment encroaches upon red, rather than melanocyte migration being responsible, phaeomelanin (red)-producing melanocytes are transformed into eumelanin (black)-producing cells by some kind of serially transmissible factor derived from contiguous eumelanotic melanocytes. By utilizing two isogenic strains (Nos. 2 and 13) of spotted guinea pigs and their F(1) hybrids, the mechanism underlying the spread of pigment from black into red skin has been analyzed, employing cellular transplantation antigens as melanocyte "markers." The findings demonstrate unequivocally that a physical migration of pigment cells is responsible. By comparing the extents of pigment spread from black ear skin grafts, or from epidermal cell suspensions prepared therefrom, from parental strain or from F(1) hybrid donors in white host skin areas of F(1) hybrid guinea pigs, it has been possible to evaluate the influence of the intimate contact of melanocytes with alien transplantation antigens on their survival and migratory behavior. No evidence was forthcoming that pigment spread takes place less readily when the cells responsible are confronted by epidermal cells bearing foreign antigens than when they are confronted by cells of their own antigenic constitution. These findings are contrary to expectation if the phenomena of allogeneic inhibition or contact-induced cytotoxicity apply to normal cells in in vivo situations.