Plasmids are constructed by using recombination in vitro according to Roberts, T.M., Kacich, R. & Ptashne, M. (1979) Proc. Natl. Acad. Sci. USA 76, 760-764 in which the t antigen gene of simian virus 40 is fused to a promoter of the Escherichia coli lac operon. In the fusions, transcription commences at the lac promoter, and, in some of the fusions, translation begins at the ATG initiator codon of the t gene. This translation is directed most efficiently by those plasmids in which the lac sequences abut the t gene such that a hybrid ribosome binding is encoded. In this case, the Shine-Dalgarno sequence is of lac origin but the ATG derives from the t gene. translation from this initiator codon is greatly decreased if the lac sequences are separated from the ATG by 17 base pairs and is abolished if the AT of this triplet is deleted. Cells bearing the productive fusions synthesize a 20,000-dalton protein with t antigenic determinants. This protein has an isoelectric point(s) indistinguishable from that of t antigen isolated from simian virus 40-transformed cells. Moreover, a partial sequence of the amino-terminal region of the bacterial product is that predicted for authentic t antigen. We conclude that these bacteria are for authentic t antigen. We conclude that these bacteria are producing a protein, the sequence of which is identical to that of authentic t antigen unfused to other polypeptides.