We found an active A enzyme in the serum of A3 phenotype. The level of enzyme activity, pH optimum, Km and Vmax values, and heat stability were not different from those of A1 gene-specified enzyme. Red cell membrane from anti-A agglutinable cells showed weak A enzyme activity, whereas those from unagglutinable cells did not. From these results, it was assumed that the weak A expression in the present case is due to the suppression of A enzyme production only in hemopoietic tissues. This special A3 phenotype is highly accumulated in the Toyama district (frequency of approximately 0.00086).