A review of the Vel blood group system reveals that 19 thoroughly studied examples of anti-Vel are known and at least 50 Vel negative persons are recorded. The frequency of the Vel negative person is 0.04 per cent. All cases to date have been in Caucasians. The antigen is poorly developed in cord blood, but retains reactivity in the aged, on storage at 4 C, as well as after freezing. An associated hemolysin is usually not reactivated by addition of complement after inactivation. Bromelin technics are best. Independence from most major blood groups is established. Clinical importance is in difficult crossmatching and transfusion reactions. No hemolytic disease of newborn has been reported (three examples of anti-Vel present during pregnancy). Family studies are essential to discover compatible donors, since 25 per cent of the siblings of Vel negative persons must also be Vel negative. The American Association of Blood Banks Central File has a list of available donors. Awareness of the existence of anti-Vel facilitates the transfusing of a patient with high frequency antibody.