The complete sequences of the SV40 agnogene (LP1) and the genes coding for the capsid proteins VP1 and VP2 have been cloned into Escherichia coli expression plasmids. High levels of expression were obtained when the SV40 genes were inserted into the coding sequence of the influenza virus NS1 gene, which has previously been expressed in E. coli. The NS1A-LP1 and NS1A-VP2 chimeric proteins consist of the 81 N-terminal residues of NS1 (designated as peptide NS1A) fused to the complete sequence of the corresponding SV40 protein. The NS1A-VP1 chimera consists of NS1A followed by a linker of nine arbitrary residues and the complete sequence of the SV40 major capsid protein. The observed levels of expression vary considerably among the three chimeric proteins, ranging from approx. 70 micrograms/ml in the case of NS1A-LP1 to approx. 5 micrograms/ml in the case of NS1A-VP2. Cyanogen bromide cleavage of the NS1A-LP1 fusion protein produces fragments with Mrs expected for isolated NS1A and LP1 peptides. A plasmid has also been constructed which expresses the NS1A peptide in high yield.