Gαq and Gα11, members of the Gq family of G-proteins, transduce signals from receptors to the β isoenzymes of phosphatidyl-inositol-specific phospholipase C (PI-PLC). The receptor specificity of these α subunits is unknown. Gαq and Gα11 are ubiquitously expressed in tissues; however, there have been conflicting reports of the presence or absence of Gα11 protein in haematopoietic cells. Platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors activate PI-PLC via Gαq, but the role of Gα11 is uncertain. To define their roles in platelet activation we studied Gαq and Gα11 gene expression by immunotransfer blotting and by reverse transcription of mRNA followed by PCR (RT-PCR) and direct sequencing. An antiserum specific for mouse Gα11 failed to identify Gα11 in dog or human platelets or in dog liver, a tissue known to contain Gα11. RT-PCR performed with gene-specific primers demonstrated Gαq mRNA, but not Gα11 mRNA, in normal human and mouse platelets and in thromboxane-sensitive and thromboxane-insensitive dog platelets. Studies of mouse and dog liver and human retina confirmed that the cDNA, primers and probes used could amplify and recognize Gα11 in other tissues. However, species-specific oligonucleotide primers and probes were essential to demonstrate Gα11, but not Gαq, mRNA. Compared with mouse cDNA, dog and human Gα11 cDNA had twice as many nucleotide substitutions (approx. 12% compared with approx. 6%) as Gαq. Gαq mRNA was also found in mature erythrocytes but Gα11 mRNA was not identified, whereas both Gαq and Gα11 mRNAs were found in bone marrow stem cells. Therefore Gα11 gene expression in haematopoietic cells is linked with cellular differentiation. The lack of Gα11 indicates that signal transduction from platelet TXA2/PGH2 receptors to PI-PLC occurs via Gαq, and that Gα11 deficiency is not responsible for defective activation of PI-PLC in thromboxane-insensitive dog platelets. Despite the high degree of similarity that exists between Gαq and Gα11, significantly greater species-specific variation in nucleotide sequence is present in Gα11 than in Gαq. Cellular specificity and species specificity are important characteristics of these Gq family G-proteins.