The capacity of young hybrid colonies between 2s rat hepatoma cells and mouse L-fibroblasts to reexpress rat albumin and become activated for mouse albumin production, was examined at the level of individual cells using immunofluorescent staining of intracellular albumin. Most of the colonies that happened to reexpress albumin were observed to present a stable, homogeneous, hepatoma-like morphology, visible long before the first signs of albumin reexpression. These colonies switch from an extinct to an albumin-producing state between one and four weeks after fusion. Colonies that do not express albumin, present an epithelial or fibroblastic morphology. Karyologic analysis of hybrid clones representative of the various morphologic types revealed that their phenotypic diversity is correlated with their rat over mouse chromosome ratio. The results suggest that the potential of hybrid colonies to express albumin is determined at the time of nuclear fusion in the heterokaryons, possibly by the number of parental genomes participating in the formation of the mother hybrid cell. Double immunofluorescent staining of rat and mouse albumin in the same cell has been used to determine whether reexpression and activation are correlated phenomena or appear independently in any single cell. The analysis demonstrates that activation of mouse albumin never takes place without reexpression of the previously expressed rat albumin gene, while the converse is frequent.