Abstract: Biologically active PDGF analogs expressed in yeast are disclosed. The analogs are produced by yeast strains transformed with an extrachromosomal element composed of a strong transcriptional promoter directing the expression of a gene which encodes a protein having substantially the same biological activity as PDGF. Suitable genes include the v-sis gene or a derivative of the v-sis gene of simian sarcoma virus or portions thereof, or the human cDNA gene for PDGF or portions thereof. A secretory signal sequence may be provided upstream of the gene, enabling secretion of the gene product from the yeast host cell. Mitogenic activity is one of the biological activities possessed by these PDGF analogs, making them useful in promoting the growth of mammalian cells.