The role of the retinoblastoma gene (RB1) in human gastric carcinogenesis is yet to be clarified. We report on the analysis of RB1 structure and protein (pRB) expression in gastric carcinomas using Southern blotting, Western blotting and immunohistochemistry. The relationship between pRB expression and cell proliferation was assessed by a proliferation marker (PCNA) in a subset of cases. Non-neoplastic mucosas were studied, as controls, by the same methodology. We found a close relationship between pRB expression and PCNA in non-neoplastic mucosas as well as in gastric carcinomas. All tumours were immunohistochemically positive for pRB, although with a variable proportion of non-immunoreactive cells. Carcinomas of the diffuse type showed absence of pRB expression in a larger proportion of neoplastic cells than carcinomas of the intestinal type (P < 0.05). Analysis of the RB1 structure using probe p68RS2.0 revealed allelic imbalance in 29% of informative cases. No homozygous deletions and/or rearrangements were detected with p68RS2.0 and cDNA probes. Western analysis revealed no abnormal patterns of pRB. Our data therefore suggest that major alterations affecting the RB1 gene are rather infrequent in human gastric carcinomas.