A recombinant plasmid (pPV1-958) carrying a poliovirus cDNA insert corresponding to nucleotides 1-5750 of the poliovirus RNA genome was constructed through in vitro recombination of two plasmids carrying overlapping poliovirus cDNA inserts (Van der Werf et al., Proc. Natl. Acad. Sci. USA 78 (1981) 5983-5987). pPV1-958 was then cleaved with PstI and the fragment carrying the sequence encoding capsid polypeptide VP1 was isolated and subcloned at the PstI site of pBR322. The VP1 sequence was successfully fused with the 5' sequence of the beta-lactamase gene by deleting 300-350 bp from each side of the PvuI site of the plasmid with nuclease BAL31. One of the resulting plasmids, pSW119, expressed an Mr 49 000 VP1-beta-lactamase-fusion protein, which was specifically immunoprecipitated with anti-VP1 immune serum but not by hyperimmune sera raised against native or heat-inactivated virus.