Human embryonic kidney cells have been transformed by exposing cells to sheared fragments of adenovirus type 5 DNA. The transformed cells (designated 293 cells) exhibited many of the characteristics of transformation including the elaboration of a virus-specific tumour antigen. Analysis of the polypeptides synthesized in the 293 cells by labelling with ³⁵S-methionine and SDS PAGE showed a variable pattern of synthesis, different in a number of respects from that seen in other human cells. On labelling the surface of cells by lactoperoxidase catalysed radio-iodination, the absence of a labelled polypeptide analogous to the 250 K (LETS) glycoprotein was noted. Hybridization of labelled cellular RNA with restriction fragments of adenovirus type 5 DNA indicated transcription of a portion of the adenovirus genome at the conventional left hand end.