Reaginic antibody synthesis after intraperitoneal immunization with ovalbumin in aluminium hydroxide gel (Al(OH)3) has been evaluated in BN, F344 and ACI inbred strains of rats and compared to that in outbred Wistar rats. Regardless of the dose of antigen employed (0.1 mg, 0.01 mg, 0.001 mg), BN inbred and Wistar outbred rats produced comparable amounts of reaginic antibody 9 days after immunization and these reagins were detectable for at least 70 days. In addition mean serum reagin titres could be boosted by subsequent administration of antigen. There was no statistically significant difference in the response of males as compared to females. The use of Al(OH)3 gel as an adjuvant yielded reaginic antibody titres in these animals that were comparable to those induced with antigen administered with saline extracts of Bordetella pertussis organisms. In contrast ACI inbred rats failed to generate detectable reaginic antibodies when immunized either singly or repetitively with the same doses of antigen in Al(OH)3 gel as used in the BN and Wistar rats. Wistar, BN, and ACI strains produced IgGa-like antibodies after a single immunization; however, peak serum titres were somewhat greater in the Wistar and BN, as compared to the ACI strain. Haemagglutinating antibody titres were comparable and greater than 1:1000 in all the strains studied within 35 days after immunization. These studies have demonstrated another animal model to explore the genetic mechanisms involved in the synthesis of reaginic antibody.