Erythrocytes, which appeared to belong to a weak A subgroup, were studied by the indirect immunofluorescence technic utilizing anti-A typing serum and fluorescein-conjugated anti-IgG/IgM. Rare, single fluorescing cells, similar to Ax control cells, were observed in preparations of erythrocytes from three members of the family studied, the father and two sons. The mode of inheritance of the weak A (Aw) gene, presumably allelic at the ABO locus, was elucidated by direct observation of fluorescent patterns in erythrocytes of family members. This technic can be useful in detecting weak subgroups of the ABO blood group system. However, it is not possible to classify these weak subgroups into specific categories, e.g., A3, Am, or Ax, using this technic, since they do not segregate into well-defined subgroups but show considerable degree of overlap. The fluorescent antibody technic, as opposed to the absorption and elution methods commonly used to identify weak subgroups, allows direct observation of the antigens present on erythrocytes. Furthermore, saliva studies used for identifying ABO subgroups can be employed only for those individuals who secrete soluble blood-group substances. Since the fluorescent antibody technic is not dependent on secretor status, it is valuable for investigating weak subgroups in these persons.