Recent evidence has suggested that the initiation of antibody synthesis is a two-step process. The first step involves antigen phagocytosis and processing by macrophages (1, 2) and the second step appears to consist of the transfer of immunogenic information from the macrophage to the immunocompetent cell with subsequent initiation of antibody synthesis (3).In most instances immunological immaturity in young or immunological deficiency in adult animals is attributed to the absence of immunocompetent cells. Recently however, it was reported by Gallily and Feldman (4) that the immunological deficiency after low doses of X-irradiation could be restored by the administration of macrophages. This suggests that the immunological insult in adult mice by low doses of X-irradiation is not directed against the immunocompetent (antibody-synthesizing) cell but is directed against the antigen-recognizing or antigen-processing cells, namely the macrophages.In this paper we present evidence suggesting that immunological immaturity in newborn mice is not due to the lack of antibody-synthesizing (immunocompetent) cells but is due, at least in part, to the lack of antigen-recognizing or antigen-processing cells, in the form of competent macrophages. This evidence was obtained by transplanting adult macrophages into newborn mice and finding that this enhanced the rate of immunological maturation to sheep red blood cell antigens. Materials and MethodsAdult C3H/He mice were purchased from the Jackson Memorial Laboratory, Bar Harbor, Me. Newborn mice were obtained from our own colony of C3H/He mice which were originally acquired from the Jackson Memorial Laboratory and maintained by brother and sister matings. Skin grafts can be freely exchanged between our C3H mice and the C3H/He mice obtained from Bar Harbor. All mice were housed in humidified, air-conditioned rooms. They were fed a Purina diet, fortified with occasional portions of dried milk and oats.Treatment of Mavrophage DoTwrs--Peritoneal macrophages were provoked in adult mice by intraperitoneal injection of 3-ml sterile thioglycoUate medium. The thioglycollate (Difco Laboratories, Detroit, Mich.) was prepared by adding 2.98 g powder to 100-ml cold distilled water and dissolved by heating to boiling point.

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