1. Transplantation of peritoneal macrophages from thioglycollate-stimulated adult C3H donor mice, into 3-day-old C3H mice results in an enhanced antibody response to simultaneously injected SRBC. The increase in immuno-competence is even more pronounced when 1-day-old C3H mice are pretreated with adult macrophages and sensitized 3 days later with SRBC. Nonviable macrophages or nonviable spleen cells are ineffective.

2. There is a critical period in the development of the neonatal mouse during which the spleen cells benefit from the addition of adult macrophages. Treatment before or beyond this stage is ineffective.

3. Very high doses (20 million) of macrophages are less effective in stimulating antibody synthesis to SRBC than doses of 5 or 10 million, suggesting that a critical ratio of macrophages to immunocompetent cells may be required for enhancing antibody synthesis in young mice.

4. The results are discussed in the light of the hypothesis that newborn mice are immunologically deficient not because they lack immunocompetent cells but because they lack an antigen recognition or antigen-processing system in the form of functional macrophages.

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