The effects of injecting monkeys with goat antiserum to IgD, the IgG fraction of that antiserum or normal goat serum (NGS) were compared. The subcutaneous injection of 4 ml/kg of the whole antiserum resulted in decreased percentages of lymphocytes with surface IgD or IgM lasting from day 1 through day 7 post-injection followed by substantial recovery on day 10. Lymphocytes from these animals were stimulated as indicated by the increased incorporation of 3H-TdR by cells placed in culture on days 7-21 post-injection. The increased blastogenesis occurred in rosette-depleted (B cell) populations and did not occur in rosette-enriched (T cell) preparations. Hypergammaglobulinaemia and increased concentration of serum IgG were first detected on day 10 postinjection, maximal on day 14 and were in decline by day 18. Injection of 4 ml/kg NGS did not alter the percentages of lymphocytes with surface immunoglobulins, result in hypergammaglobulinaemia, or stimulate the degree of blastogenesis observed after anti-IgD. Injection of the IgG fraction of the antiserum resulted in decreased lymphocytes with surface immunoglobulins but did not stimulate hypergammaglobulinaemia or increase blastogenesis. Injection of one monkey with the IgG fraction of anti-IgD combined with NGS resulted in increased serum IgG and increased blastogenesis. Both antibody to IgD and multiple antigenic challenge appear to be required for these responses.