S13 is an avian retrovirus that transforms both fibroblasts and erythroblasts. The gene product responsible for the oncogenic effects of S13 is the env-related glycoprotein gp155. In this report we show that gp155 is a transmembrane protein with a 55-kDa cytoplasmic domain. Pulse-chase analysis shows that gp155 was cleaved posttranslationally into two glycosylated proteins, gp85 and gp70. In addition, we show that a tyrosine protein kinase activity is associated only with the gp70 protein in microsomes and in immune complexes.