After infection with 10(3) T. brucei GUTat 3.1, C57BL/6 mice produced antibody responses and controlled the first parasitaemic wave whereas C3H/He mice did not. The inability of C3H/He mice to control parasitaemia resulted from an impaired ability of parasite-induced antibody-containing cells to secrete immunoglobulin. Antibody-containing cells in infected C3H/He mice regained the ability to secrete antibody within 24 h after trypanosome elimination by treatment with Berenil, suggesting that the block in antibody secretion was maintained by living parasites or short-lived components of degenerating parasites. Infected C3H/He mice also had an impaired ability to produce a rabbit erythrocyte-specific antibody response on challenge with rabbit erythrocytes and this response recovered when parasites were eliminated from the blood 24 h before analysis. It was not possible to inhibit secretion of antibody by rabbit erythrocyte-induced plasma cells either by incubating them with serum from infected C3H/He mice or by injecting large numbers of living trypanosomes into C3H/He mice already responding to rabbit erythrocytes. The process leading to failure of parasite and rabbit erythrocyte-induced antibody-containing cells to become high rate antibody-secreting cells was not identified but did not appear to correlate with any obvious change in the intra-cellular morphology of the antibody-containing cells.