Normal human colon mucosal epithelial cells were cultured in vitro and treated with the oncogenic simian DNA virus (SV40) and the chemical carcinogen azoxymethane. Both SV40 and azoxymethane altered a number of phenotypic characteristics of the normal human colon cells, including their morphology, culture longevity, growth in soft agar, substrate adherence, and peanut agglutinin binding. The SV40 transformants synthesized intranuclear T antigen. These data indicate that normal human colon mucosal cells were transformed toward the malignant phenotype.