Three capsid proteins oi' SV40 (VP1, VP2, and VP3) were expressed in insect cells using recombinant baculoviruses. When the VP1 eupskl protein was expressed alone or co-expressed with VP2 and VP3, virus-like particles (VLP) were produced. In the latter case, the minor capsid proteins, VP2 and VP3, were incorporated into the VLP. VLPs with and without VP2 and VP3, and the wild type SV40 virions were indistinguishable under electron microscope. The sedimentation coefficient, v-,Uw, obtained for the VLP consisting of VP1 iilone (VPI-VI.P) was 170 S, and that for the VLP consisting of all of the capsid proteins (VPt/2/3-VLP) was 174 S. Treatment of the VI11 VLP with a calcium ion dictating agent and a reducing agent caused dissociation of the VP1 -VLP. The dissociated and purified VP1 pmieins were identified as pentamers of VP1 based on the molecular weight determination by sedimentation equilibrium. The pentamers were shown to possess the ability to re-assemble into VLP which had the s20w of 141S. The results are discussed in relation to the morphogenesis of SV40.