The IgG1 molecules in the sera of IgG1 hypergammaglobulinaemic mice chronically infected with the larval cestode, Mesocestoides corti, are a heterogeneous population. Although antibodies to M. corti are present, the question of whether a minority or majority of the serum IgG1 molecules has anti-parasite reactivity remains open. The splenic PFC response to an intravenous injection of SRBC in M. corti-infected mice does not consist of an unusually high proportion of IgG1 anti-SRBC PFC. Moreover, the adoptive anti-DNP PFC response of spleen cells from M. corti-infected mice to DNP-M. corti is not biased towards IgG1 antibody production. Since IgG1 hypergammaglobulinaemia is seen in mice with chronic, "high-dose" infections, an attempt has been made to simulate chronic antigenic exposure with SRBC in uninfected mice. A split, high-dose regime of SRBC injections leads to a high number and high proportion of IgG1 anti-SRBC PFC in the spleen in three strains of mice. The results suggest that the extraordinarily high levels of IgG1 seen in the sera of mice chronically infected with the metazoa, M. corti and Nematospiroides dubius, reflect persistent, high-dose, "strong", T cell-dependent stimulation of the B cell system.