Previous work has shown that proteins phosphorylated on tyrosine are selectively detectable by antibodies against phosphotyrosine (P-Tyr) in cells transformed by retroviral class-1 oncogene-encoded kinases endowed with non-regulated activity (Di Renzo et al., 1986). In this work P-Tyr antibodies were used to investigate the existence of human tumors expressing abnormal levels of tyrosine phosphoproteins and tyrosine kinases. Among 18 cell lines examined, the antibodies identified a number of tumors with a detectable level of proteins phosphorylated on tyrosine. Among these were a major protein with an approximate Mr of 150,000 in a gastric carcinoma; 2 proteins, with Mr of 130,000 and 110,000 in a colon carcinoma; a major protein with Mr of 170,000, tyrosine phosphorylated in both a urinary bladder and an epidermoid carcinoma; a 100,000 Mr protein phosphorylated in lung and breast carcinomas. An 80,000 Mr tyrosine phosphorylated protein was found in a fibrosarcoma and in a rhabdomyosarcoma. Among the hemopoietic malignancies screened, in 2 Philadelphia-positive chronic myelogenous leukemias P-Tyr antibodies recognized the chimeric bcr-abl 210,000 Mr protein and its substrates. Two tyrosine phosphorylated proteins, one of Mr 70,000 and one of Mr 60,000, were detected in a Burkitt lymphoma line. These phosphoproteins were not found in samples harvested from normal gastro-intestinal or urinary bladder epithelium, nor in control fibroblasts and lymphocytes. Two of the above proteins have associated tyrosine kinase activity: the 170,000 Mr protein of bladder carcinoma cells was found to be a constitutively phosphorylated EGF receptor.