Abstract The immune response of the gar (Lepisosteus platyrhincus), a fresh water holostean, was investigated to test its capacity to evince immunologic memory and to characterize the molecular species of antibody produced during prolonged immunization (414 days). With aluminum phosphate-adsorbed diphtheria toxoid, the response to primary stimulation was poor. In contrast, after secondary stimulation all fish showed a uniform anamnestic response with antibody persisting for at least 275 days. The latter response also was characterized by the presence of precipitating as well as hemagglutinating antibody. With the use of alum-precipitated bovine serum albumin, a variable degree of secondary response was noted depending upon the dose of antigen, the best results being obtained with 5 mg and 50 mg. With Salmonella typhosa flagellar antigen a secondary response could not be detected, possibly owing to the high titer of “natural” antibody present in non-immunized fish. Antisera obtained during the course of immunization with each of the antigens were subjected to gel filtration on Sephadex G-200 and to density-gradient ultracentrifugation. The antibody activity was seen to reside only in the macroglobulin fraction. Fractions obtained by gel filtration of gar sera from the secondary response to BSA also were assayed by the Farr technique. Radioactive binding was detected only in the fractions from the macroglobulin region. Radioimmunoelectrophoresis with 131I BSA showed one arc of radioactivity, which corresponded to the arc of precipitation formed at the origin when developed with BSA. Antisera to all antigens were wholly susceptible to mild reduction and alkylation. In summary, the gar were found to manifest an anamnestic response that was associated exclusively with macroglobulin antibody.