The expression of epithelial features has been investigated from the 9th to the 30th passage in cells derived from the fusion of mouse 3T3.4E cells with normal or hand wart human keratinocytes. These cells stratified and grew either on plastic or collagen substrate without modifications. Such growth characteristics are similar to those observed in the epidermis. The doubling time of hybrid cells is nearer to that of 3T3.4E than to that of keratinocytes. Some epithelial markers were detected by immunofluorescence staining. Bullous pemphigoid antigen characteristic of the basal keratinocyte was detected, and cytoplasmic antigen of basal cells was positive with BL7 monoclonal antibody. Among the antigens of the suprabasal cell layers of the epidermis, the pemphigus antigen was present in all hybrids; epidermal keratins recognized by the monoclonal antibodies KL1 and KL2 were detected in wart hybrids up to the 9th passage. When hybrids were cultured in delipidized serum or in methylcellulose, few cells (less than 10%) reacted with KL2. Karyological analysis revealed both murine acrocentric and human submetacentric chromosomes. Thus, hybrid cells obtained from normal and wart keratinocytes were new cell types with a phenotype between keratinocytes and 3T3.4E cells.