Subcellular fractionation of rat and human cells transformed by the adenovirus type 12 (Ad-12) EcoRI-C DNA fragment showed that the 41000 mol. wt. (41K) E1a and 52K E1b proteins were present in the nucleus and cytoplasm at approximately equal concentrations. The 18K E1b protein was associated with the nuclear, mitochondrial, lysosomal and membrane fractions. The 41K E1a protein was also associated with various cytoskeletal structures (probably microtubules and 10 nm filaments) in Ad-12-transformed cells. The Ad-12 E1 41K and 52K proteins have been partially purified from transformed and infected cells. Using these preparations the 52K protein has been shown to exist under non-reducing conditions and probably in vivo as a 100K dimer stabilized by intermolecular disulphide bonds. The 41K protein bound strongly to histones H1 and H4 but much more weakly to H2A, H2B and H3. It did not interact with other comparable basic proteins or with the cytoskeletal components actin, tropomyosin and calmodulin. Although the 41K E1 a protein bound to histones in vitro it is probable that such an interaction may not occur in vivo as very little of the adenovirus protein co-purified with chromatin from transformed cells. None of the Ad-12 E1 proteins showed any ATPase or protein kinase activity.