The serums of relatives of male children with hypo- gammaglobulinemia were examined for the content of immunoglobulins. In three instances the concentration of IgM was low in the fathers and in two of these cases the same abnormality was also found in the paternal grandfathers. In a fourth family the father had general- ized hypogammaglobulinemia and the paternal grand- mother had a low concentration of IgM. Abnormalities were not found in the mothers. These observations suggest a role for paternal genetic factors in the inheritance of some forms of hypogam- maglobulinemia. Although the number of cases studied to date is small, it is possible that these abnormalities may identify the male carrier of this disorder and facili- tate genetic counseling.