Immunohistochemical study of the expression of simple mucin-type carbohydrate antigens (Tn, sialosyl-Tn, T and sialosyl-T) was performed using specific monoclonal antibodies in the mucosa adjacent to gastric carcinomas histologically appearing unaffected (n = 58), and in primary gastric carcinomas (n = 87) and their metastases (329 lymph nodes and two liver metastases). Normal-looking mucosas: Tn antigen expression was identified in all the cases; sialosyl-Tn in eight cases; T antigen was never expressed and sialosyl-T antigen was observed in four cases; the expression of these antigens was distinctly limited to the cytoplasm, mostly in the supranuclear (Golgi) area. All the mucosas with intestinal metaplasia showed sialosyl-Tn expression in the goblet cells. Gastric carcinomas: 80 cases (91.9%) stained for Tn antigen, 69 cases (19.3%) expressed sialosyl-Tn antigen, 18 cases (20.7%) expressed T antigen and 17 cases (19.5%) stained for sialosyl-T antigen. In contrast to normal mucosa, carcinoma cells expressed simple mucin-type antigens both at the cytoplasm and at the cell membrane. Most primary carcinomas were concurrently stained for Tn and sialosyl-Tn antigens alone (41.1%), or together with T antigen or sialosyl-T antigen (28.7%). We found a close relationship between the expression of simple mucin-type carbohydrate antigens in the primary tumours and their respective metastases. T antigen (and sialosyl-T antigen) expression was correlated with the wall invasiveness of the tumours. The 18 tumours expressing T antigen and 16 out of the 17 tumours expressing sialosyl-T antigen had nodal metastases and/or sialosyl-Tn expression and the aggressiveness of the tumours (wall penetration, lymph node metastasis and venous invasion). No significant differences were observed between positive and negative tumours for Tn, sialosyl-Tn, T and sialosyl-T antigens regarding the morphologic at type, growth pattern, ploidy or lymphoid infiltrate of the primary tumours.