Postnatal serum concentrations of IgG2a of paternal allotype, measured in congenitally thymusless nude mice, increase with kinetics and titers comparable to their normal congeneic counterparts. Lipid A, the mitogenic part of LPS, stimulates IgG synthesis in nude mice when it is given 7 days after birth. IgG concentrations at 15 days of age are 6- to 8-fold higher than in untreated control nudes; this is considerably lower, however, than in normal mice, which show up to 45-fold higher IgG2ab levels after lipid A treatment. A thymus graft from nearly congeneic donors of the same age, transplanted at 4 days after birth, also stimulates long-lasting IgG synthesis in the nude recipients. If the grafted nudes are injected with lipid A 3 days later, IgG synthesis is further stimulated 8- to 16-fold. The data are discussed in relation to the thymus dependency of IgG production and the conditions for lipid A stimulation.