We have characterized a family of transplantable melanomas in Syrian (golden) hamsters, which originated in 1959 as a spontaneous melanoma of hamster skin, and which has been maintained since then by serial passage. Emphasis has been placed on using the same method of transplantation, keeping strict records on all passages, and applying the same investigative techniques, in order to trace tumor behavior over long periods of time. This tumor family consists of five variants linked by common origin, but which differ with respect to differentiation level, malignancy, intermediary metabolism, chromosome number, and cell surface properties. Once established, these melanomas possessed a considerable degree of phenotypic stability over decades of passaging. One tumor line in this family is emphasized. The Ab amelanotic melanoma lost its differentiated functions (the ability to synthetize melanin) a quarter of a century ago, and since then has remained dedifferentiated in serial passage in hamsters. After transfer to primary cell culture, the Ab melanoma cells differentiate readily and lose much of their proliferative potential. This process is reversible by reimplantation of the cells into a hamster. Inasmuch as this hamster melanoma system meets many of the conditions required for an experimental tumor model, five melanoma variants are characterized concisely and compared to other melanomas in humans and animals. Mechanisms by which new melanoma variants arise are discussed and compared to some phenomena in the evolution of the species.