Polyclonal antibodies to the human retinoblastoma gene product (Rb1) have been produced in rats by immunisation with a fusion protein comprising part of Rb1 together with the E. coli beta-Gal sequence. We have used these antibodies in Western blotting studies to screen a number of human foetal tissues and organs and found approximately similar levels of expression of Rb1 in all of them. The protein seems to be somewhat more abundant in some cell lines produced by transfection of human embryo retinal (HER) cells with adenovirus 12 early region 1 (Ad 12 E1), Ad 5 E1, Ad 2 E1A + mutant N-ras or SV40 DNA. Using co-immunoprecipitation followed by Western blotting we have shown that the Rb1 protein binds to Ad 12 E1A 266 and 235 amino acid proteins. This interaction is ionic strength dependent but is unaffected by non-ionic detergent up to a concentration of at least 1%. In Ad 12 infected human cells it appears that less E1A is bound to Rb1 than in the transformants. These results are discussed in view of the known similarities and differences between the amino acid sequences of Ad 12 and Ad 5 E1A proteins.