The expression of proteins encoded by ras oncogenes was examined in 52 fresh human tumors of 19 different types using antibodies generated to different peptide domains of ras proteins of molecular weight 21,000 (p21). Proteins related to ras genes were detected in 90% of tested tumors. In 21% of tumors there were high levels of ras p21 of greater than 40% of the level in a Harvey murine sarcoma virus transformed cell line. Predominance of either cellular Ha-ras or other ras p21s was found in 20 and 14% of tumors, respectively, and predominance of p21 other than Ha-ras was frequent in breast cancers. Abnormal electrophoretic mobility of p21 was observed in two cancers, and a novel rapidly migrating ras p21 was found in a rare malignant fibrohistiocytoma.