The v-src gene of Rous sarcoma virus (RSV) encodes pp60v-src, a tyrosine kinase that can initiate cellular transformation. High levels of v-src gene expression can either be cytotoxic or the cause of altered expression of cellular genes. Examination of cytotoxic thresholds is difficult because cells expressing high levels of a cytotoxic oncogene will die. To evaluate quantitatively the cytotoxicity of pp60v-src on growth, we amplified two different v-src genes, under the control of the human hsp70B heat shock promoter to establish cell clones with varying copy numbers of the heat-inducible v-src gene. The viability of cell lines over a prolonged period of time depended on the particular src gene, the expression of v-src mRNA, synthesis of the pp60v-src protein and, most importantly, the tyrosine kinase activity of the pp60v-src protein. We found a relatively sharp threshold in v-src-encoded tyrosine kinase activity above which cell viability rapidly declines. However, over time, tyrosine kinase activity was exponentially suppressed at about a 10-fold higher rate than pp60v-src protein during passage. Our results indicate that homeostasis of tyrosine phosphorylation is important for cell viability, that perturbation of this balance results in cell mortality, and that cells can evolve to accommodate overexpression of oncogene by downregulating the level of tyrosine kinase activity.