The main conclusion from this study is that C3 receptors are not required for the generation from B cells of a thymus-dependent 7S antibody response. The requirement for C3 receptors on the precursors of antibody-forming cells was studied in an adoptive transfer system using thoracic duct lymphocytes (TDL) from primed rats as a source of precursors and irradiated recipients as hosts. 7S precursors were found in both the CR+ and the CR- fractions of TDL and it was established that the response transferred by CR- cells did not arise from either a raidoresistant B cell in the host or from CR+ cells contaminating the CR- population. Thus, the C3 receptor is not obligatory for B-cell-T-cell cooperation in the 7S response. The precursors of 19S antibody-forming cells were found only in the CR+ subpopulation. The CR-Ig+ subpopulation was shown to contain all the B blasts in rat TDL and a very small number (approximately 1% of all TDL) of small lymphocytes. This latter population contained the CR- 7S precursors and contributed approximately 20% of the total adoptive secondary 7S response transferred by CR+ and CR- subpopulations combined. This observation suggests that the percentage of rat TDL committed to carry 7S memory is small, a conclusion which is confirmed and extended in the following paper.

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