The naturally occurring hemolytic plaque-forming cells (N-PFC) against sheep red blood cells (SRBC) are found in several lymphoid organs of adult mice, with the majority being found in the spleen. Only N-PFC of 19S type were found which behaved like the 19S PFC arising after immunization in all systems tested except one: the N-PFC were found to be completely refractory to inhibition by passively administered anti-SRBC antibodies. The possible reasons for this discrepancy are discussed.

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