Laboratory and field tests were conducted at Churchill, Manitoba, in 1978 and 1979 to determine whether human menstrual odors attract polar bears (Ursus maritimus). The polar bear was chosen because the odor of its primary prey, seals, could be used as a standard attractant against which the relative attractiveness of other scents could be measured. When 4 captive bears were presented with a series of different odors, a strong behavioral response was elicited only by seal scents and menstrual odors (used tampons). When these materials were arrayed in a location frequented by free-ranging bears, the bears detected and consumed, the food scent samples and used tampons, but ignored the non-food scents and human blood. The primary method of detection was by scent with a minority of the samples being detected visually or by other means. The lack of response to the other scents, combined with the fact that in the laboratory and field the bears' responses to seal scents demonstrated that menstrual odors attract polar bears. Comparing bear responses to human blood and menstrual blood indicates that it is not the blood that is acting as the attractant, but some property peculiar to the menstrual products.