The baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) has been genetically manipulated to yield a recombinant virus capable of expressing p24, the major core protein of HIV-1, in insect cell culture. The expressed product is a p24 protein flanked by short regions of p17 at the amino terminus and p12 at the carboxy terminus. It has been identified and characterized using monoclonal antibodies on Western blots and by amino-terminal sequence analysis. The presence of p24 in the soluble fraction of infected cells following lysis by detergent or sonication, combined with a high level of expression (in excess of 50 mg/l of culture) facilitates the enrichment of large quantities of recombinant HIV antigen in a simple two-step procedure involving ammonium sulphate fractionation and gel filtration. p24 antigen purified in this way is shown to be an efficient diagnostic reagent.