FemaleSchistosoma mansoni and their excretory-secretory (ES) products were extracted with a series of solvents to provide fractions of varying polarity. These fractions were assayed for chemoattractivity to males in vitro. One major component of these mixtures was found to be nonattractive and was identified as cholesterol.n-Pentane- and ether-soluble fractions derived from ES products exhibited chemoattractive activity comparable to that possessed by whole-worm extracts, but appeared to be simpler mixtures.