The human gastric cancer cell line MKN 45 was derived from the tumour of a blood group A individual, and was known to express large quantities of blood group A-antigen. Using immunofluorescence we found the MKN 45 cells, donated from the Japanese Cancer Research Resources Bank, consisted of A-antigen positive cells (18%) and A-antigen negative cells (82%). After limiting dilution, wild type and mutant cells were cloned with regard to the expression of a cell surface A-antigen. ELISA was used to detect A-antigen in the cell extract of the wild type cells, but none was evident in those of the mutant cells. However, blood group A-gene-specified transferase activity of the mutant cells was comparable to that of the wild type cells.