It has been found that neuroblastoma contains a relatively high immunoreactivity for the alpha subunit of guanine nucleotide-binding protein G(o) (G(o) alpha) (Kato et al., Cancer Res. 47, 5800-5805, 1987), which is predominantly localized in the nervous tissues and neuroendocrine cells. To make clear whether neuroblastoma indeed produces G(o) alpha, we purified a guanine nucleotide-binding protein from human neuroblastoma and compared it with G(o) alpha from human brain. A guanine nucleotide-binding protein serving as the specific substrate of islet-activating protein, pertussis toxin, was purified from a human neuroblastoma tissue obtained at surgical resection. The protein had a molecular mass of 39 kDa and reacted with rabbit antibodies to bovine G(o) alpha. Heat stability and kinetics of guanosine 5'-(3-O-thio)triphosphate binding of this protein were very similar to those of G(o) alpha purified from human brain. The results of peptide mapping analysis with a limited proteolysis and amino acid analysis indicated that there was no difference between these proteins. Therefore, it was concluded that this guanine nucleotide-binding protein isolated from neuroblastoma was very similar, if not identical, to brain G(o) alpha.