The v-sis oncogene of simian sarcoma virus encodes a protein which is homologous to the human platelet-derived growth factor B-chain. The v-sis protein undergoes a series of processing steps including dimer formation and proteolytic digestion to generate several molecular sizes of the protein. Two of these v-sis proteins were expressed alone or as polyhedrin-sis fusion proteins using the Bombyx mori nuclear polyhedrosis virus vector. The polyhedrin-sis fusion proteins contained a collagenase-sensitive site at the junction. The expression levels of the fusion proteins whose polyhedrin portions consisted of only 8 amino-terminal amino acids were 3-4 times higher than those of non-fusion proteins. One of these fusion proteins was expressed in silkworm larvae and the v-sis protein was isolated from the fusion protein by collagenolysis followed by chromatography. Because the purified v-sis protein exhibited the same molecular size on SDS-polyacrylamide gels under reducing and non-reducing conditions, it was concluded to be monomeric in structure. It possessed chemotactic activity but lacked mitogenic activity. In addition, a small amount (approximately 1%) of monomeric v-sis protein was converted in vitro to the mitogenically active v-sis protein, which could be a homo-dimer.