Spontaneous responses to the scent marks of male Saguinus fuscicollis were tested in experiments where the subjects received a choice between scent of intact males and castrated males. Male tamarins discriminated between the odors of intact and castrated males, both on the basis of fresh individual scent marks and on the basis of marks from several donors pooled in methanol. Their discriminatory response to pooled odors was more pronounced than that to individual odors. Females discriminated between intact and castrated male scents only on the basis of pooled samples. This result shows that castration causes changes in the odor of males. The difference in the response of the tamarins to individual scent marks as compared to pooled samples is discussed in terms of quantitative changes caused by extraction and of individual recognition.