We have previously described pp60c-src SH2 mutants that are host-range-dependent for cell transformation; most of these mutants can transform CEF cells but not NIH3T3 cells, and other transform NIH3T3 cells more efficiently than CEF (Hirai and Varmus, 1990c). In an attempt to understand the molecular basis of these phenotypes, we analysed the ability of mutant SH2 domains in GST fusion proteins to bind to tyrosine phosphorylated proteins in lysates from CEF and NIH3T3 cells. The relative affinity of mutated versions of the SH2 domain for phosphotyrosine-containing proteins from CEF and NIH3T3 cells was compared with the relative ability of the mutant Src proteins to transform the two cell types. While the affinity of the SH2 domain for phosphotyrosine-containing proteins was closely correlated with transformation in NIH3T3 cells, there was no correlation between phosphotyrosine binding and transformation of CEF cells, and none of the host range mutant SH2 domains showed significant differences in their ability to bind phosphotyrosine-containing proteins from lysates from either cell type. In addition, the SH3 domain was shown to augment the capacity of mutant SH2 domain to bind phosphotyrosine-containing proteins.