Abstract Primary and secondary immune responses to flagellin were examined in recipients of renal allografts immunosuppressed with azathioprine and prednisolone, and in matched controls. The primary response, predominantly IgM, was similar in patients and controls. However, the secondary response, predominantly IgG in the controls, was suppressed in the patients after the second challenge with flagellin. In the patients there was no increase in titre of antibody when the interval between injections was short (3 months), and a primary rather than a secondary type response occurred when the interval was longer (12 months). The findings point to interference with immunological memory as one effect of immunosuppressive treatment.