Our previous studies have demonstrated that 7 of 10 IgG antibodies against distinct epitopes on the extracellular domain of the c-erbB-2 gene product (p185) inhibit the anchorage-independent growth of SKBr3 human breast-cancer cells that overexpress this transmembrane tyrosine kinase growth-factor receptor. Two of 7 growth-inhibitory antibodies also block the binding and function of the gp30 and p75 c-erbB-2 ligands. In this report we have studied phosphorylation of p185 and different intracellular substrates after binding of antibodies that do or do not inhibit tumor-cell growth. A correlation has been found between antibodies that inhibit growth and the intensity of tyrosine phosphorylation of p185. At late intervals, serine phosphorylation of at least 3 intracellular substrates is increased preferentially by growth-inhibitory antibodies. To test the importance of p185 kinase activity more critically, NIH3T3 cells were transfected with an expression vector containing the full-length human c-erbB-2 gene (cell line 17313), c-erbB-2 with deletion of the kinase region from codons 751-979 (cell line 9309) or c-erbB-2 with deletion of most of the intracellular domain from codons 684-1255 (cell line 9310). Unconjugated antibodies inhibited anchorage-independent growth of 17313 cells as well as SKBr3 cells, but did not inhibit growth of either 9309 or 9310 cells. In contrast, the cytotoxic effect of anti-p185-ricin A chain (RTA) conjugates was comparable for 17313, 9309 and 9310. The tyrosine-kinase activity of p185 is required for growth inhibition mediated by unconjugated anti-p185 antibodies, but not for the cytotoxic activity of anti-p185-RTA immunotoxins.