Bovine females produce an estrous pheromone(s). Because only other cattle are reliable detectors of the pheromone(s), we evaluated various body fluids (cervico-vaginal mucus, serum and vulval skin gland secretion) with a bull behavioral assay in the absence of a female. It specified 15 behaviors involving attraction to the body fluids, detection of the stimulus, and subseqeunt sexual preparation responses. These behaviors were stereotypical and occurred in a specific sequence. We discovered that serum (10-15 ml) taken at estrus was behaviorally positive, as well as vulval skin glands secretion (<1 ml on a swab, dissolved in 5 ml distilled water). We postulate that vulval skin glands may be a specialized site for the production or concentration of the pheromone or that the blood transports the pheromone(s) either from vulval skin glands or another source. © 1989 IRL Press at Oxford University Press.