Immunization of outbred Hartley guinea pigs with repeated minute doses (1.0 µg) of a hapten-protein conjugate, benzylpenicilloyl-bovine γ globulin, in alumina gel adjuvant, resulted in the production of both reaginic and 7Sγ1 homocytotropic antibodies specific for the hapten and for the protein carrier. Reagin was distinguished from 7Sγ1 antibody by its long persistence in homologous skin sites, its lability to heat at 56°C and its relative non-absorbability by a rabbit anti-guinea pig 7Sγ1 antiserum. The reaginic responses were high titered, persistent for several months and subject to striking booster responses to repeated minute doses of antigen. These reaginic responses were associated with the production of 7Sγ1 antibodies and relatively small amounts of precipitating antibody. By contrast, immunization of guinea pigs with 100-µg doses of the antigen in complete Freund's adjuvant resulted in the production of large amounts of precipitating antibody and only traces of reagin.