Female BALB/c mice were immunosuppressed with a series of neonatally initiated rabbit anti-mu serum injections, which eliminated serum IgM and greatly delayed production of antibodies against normal rabbit serum (anti-NRS). Females thus prepared maintained circulating anti-mu levels for several months. Study of the progeny from pregnancies completed during this maintenance period revealed that rabbit anti-mu antibodies readily cross the murine placenta but are not passed in murine colostrum at levels detectable by the technique used. Anti-NRS antibodies actively produced in NRS-injected control females do cross the placenta, but do so only irregularly and poorly; these antibodies may, however, be detected consistently at relatively low levels in colostrum. Suppression of humoral immunoglobulin synthesis in most mice prenatally exposed to anti-mu antibodies by transplacental passage appeared complete, even including loss of the remnant IgG levels which are consistently seen in mice first exposed to anti-mu at birth. The appearance of serum IgG and anti-NRS antibodies along with the complete absence of serum IgM in mice recovering from suppression suggests that active IgM synthesis and secretion may not be a prerequisite for the IgM to IgG "switch". Immune recovery occurred even in completely immunosuppressed mice after anti-mu injections were discontinued; the mechanism of recovery is not certain.