Several cellular proteins, including polypeptides of 300, 107 and 105kDa, associate with the products of the E1A gene of adenovirus type 5. Here we show that the 105kDa species is the product of the recessive oncogene Rb1 which is absent or altered in retinoblastomas and other human cancers. About 75% of the total Rb1 protein in infected human KB cells was found to be complexed with E1A products and these data support the hypothesis that E1A-mediated transformation results from the elimination of functional Rb1 protein within the cell. However, while the interaction of E1A products with this cellular protein is necessary, transformation appears also to require binding of the 300 and 107kDa polypeptides. We also found that both the Rb1 protein and the 107kDa E1A-binding species were undetectable in a retinoblastoma cell line lacking both alleles of the Rb1 gene. These data suggest that the absence of both the 107kDa and 105kDa/Rb1 proteins may be involved in the development of the oncogenic phenotype of retinoblastoma cells.