The avian retrovirus S13 codes for an env-linked transformation-specific glycoprotein with a molecular weight of 155,000 (gp155). Treatment of gp155 with endoglycosidase H or growth of S13-infected cells in the presence of tunicamycin reduces the molecular weight of gp155 to about 140K, but these gp155-related molecules may still contain sugar residues. The gp155 protein is not incorporated into virions; it is phosphorylated, but in immunoprecipitates does not show protein kinase activity. The genome of S13 is an 8.5-kilobase (kb) RNA; the helper virus genome is 7.5 kb in size. The putative onc sequences of S13 do not hybridize to DNA probes representing src, erb A, erb B, myc, myb, fps, fms, H-ras, B-lym, abl, rel, and ets.