We have identified a C to T mutation at nucleotide No 314 in the coding region of the human alpha (1,3/1,4) fucosyltransferase III (FT-III) gene held responsible for synthesis of Lewis a and b antigens. The mutation gives rise to an additional cleavage site for restriction enzyme NlaIII and was found in both alleles in 5 of 18 Lewis negative but in none of 22 Lewis positive individuals. Heterozygous individuals were found both among Le negative (10 of 18) and Le positive (4 of 22) individuals. The mutation corresponds to a threonine to methionine substitution and was found by partial sequencing of the coding sequence of one of five Lewis negative individuals homozygous for this transition.