The conditioned enhancement of natural killer (NK) cell activity can be blocked by the injection of sodium carbonate solution prior to the association of the camphor odor conditioned stimulus (CS) with the polyinosinic: polycytidylic acid (poly 1:C) unconditioned stimulus (US). We have experimentally dissociated the memory which is formed for camphor odor from that developed for the association of camphor with the US. The memory for the odor of camphor can be allowed to develop one day before the administration of the unconditioned stimulus. Sodium carbonate appears to act within the central nervous system to block the association of the unconditioned stimulus with the conditioned stimulus. The data also suggest that in conditioning of NK cell activity, there are two specific sites for memory of the response, one for the conditioned stimulus and the other for the association of the unconditioned stimulus with the conditioned stimulus. © 1994 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.