A group of membrane-associated guanine nucleotide binding proteins (G-proteins) are essential for transducing signals generated at cell-surface receptors into changes in cellular function and metabolism. These proteins are a complex of three subunits designated alpha, beta and gamma. The alpha-subunit is responsible for binding guanine nucleotides and seems to be characteristic of each protein. Transducin, a member of this protein family, mediates visual transduction by coupling the signal of photolysed rhodopsin with activation of a cyclic GMP phosphodiesterase. We have now cloned and sequenced the complementary DNA encoding the alpha-subunit of bovine retinal transducin and from this we have deduced the complete amino-acid sequence. The transducin alpha-subunit shares several homologous amino-acid sequences with ras gene products. The homologous segments correspond mostly to the regions thought to be involved in the guanine nucleotide binding and GTPase activity of ras proteins and to the ADP-ribosylation sites of the transducin alpha-subunit.