Fourteen cis AB blood samples belonging to 6 families from Belgium, France, Japan and Germany were studied with quantitative agglutination and fixation assay. Results showed that ABH antigens of cis AB erythrocytes significantly differ from "trans" AB controls: the H amount is higher than in trans AB and similar to that of A2 controls; the A amount is similar to that of A2 cells; the number of A sites per cell as measured with a 125-I labelled IgG anti-A from a rabbit ranges from 170 000 to 580 000 in cis AB samples; the A1 reactivity seems to be related to the A antigen density of erythrocytes more than to a specific A1 antigen; the B reactivity is quite normal with immune anti-B sera but weak with naturally-occurring anti-B from A individuals. The reaction of B antigen of cis AB cells with a naturally-occurring anti-B from an A1O person exhibits a weaker enthalpy change than in trans AB cells. Most cis AB sera have anti-B activity, essentially at 4 degrees C. In saliva A and H substances are found in normal amounts but B substance is only evidenced by inhibition of autologous cells agglutination. Data obtained from this study do not favour any proposed hypothesis to explain the cis AB gene formation (crossing-over between A and B genes or A gene mutation).