Tyrosine phosphorylation exerts a pivotal role in cell regulation processes of higher eukaryotes. Tight control of the activity of protein tyrosine kinases is crucial for ordered phosphorylation to occur. We have developed a functional screen for tyrosine kinase regulators using c-Src, the first cellular protein tyrosine kinase described, as a prototype; and fission yeast, Schizosaccharomyces pombe, as a genetically amenable host system. Inducible expression of c-Src in fission yeast is lethal. We have screened human cDNA libraries for clones able to counteract the lethal effect of Src. Two different classes of cDNAs, which we called SAS for sequences antagonizing Src, were obtained. The first class encodes for the protein tyrosine kinase Csk, known to regulate Src activity through phosphorylation of the C-terminal tyrosine. The second class consists of clones encoding three different tyrosine phosphatases, counteracting Src action by dephosphorylation of Src substrates and by dephosphorylation of Src itself. The system described here can be applied to identify regulators of other heterologous tyrosine kinases, including receptor-type tyrosine kinases, which impair growth of S. pombe.