About 20% of human subjects are specifically anosmic to the odor of the heterocyclic Schiff's base 1-pyrroline. Odor threshold measurements on 33 nitrogenous bases were made with panels of specific anosmics and normal observers. The anosmia proved to be very selective, and was encountered only with 1-pyrroline and its homolog 1-piperideine. The odor of 1-pyrroline resembles very closely that of human semen, and possibly represents a vestigial pheromone. It is suggested that this specific anosmia corresponds with the absence of a new olfactory primary, the “spermous” odor. The same odor occurs as an impurity in several organic reagents and can be detected in a variety of biological materials.