We have applied the polymerase chain reaction (PCR) and single-strand conformation polymorphism analysis (SSCP) to detect activating mutations in the Gs alpha subunit gene, amplifying genomic DNA extracted from growth hormone (GH)- and GH/prolactin (PRL)-secreting human pituitary tumors. Of 15 tumors tested six contained mutations in the analyzed regions of the Gs alpha. SSCP analysis revealed band shift in exon 8 in four GH- and in one GH/PRL-secreting tumors, and in exon 9 in one GH/PRL-secreting tumor. Direct sequencing of PCR reaction products identified the mutations as R201-H, R201-S and R201-C in exon 8 and Q227-L in exon 9. These results show the efficacy of PCR/SSCP analysis in the detection of G protein mutations and extend the generalization that these sites are hot spots in tumor-inducing mutations.