We have shown previously that partially purified human chorionic gonadotrophin (hCG) preparations inhibited the specific binding of 125I-labelled hLH or hCG to Candida albicans membranes at much lower concentrations than did highly purified hLH or hCG preparations. We now describe the characterization and partial purification of a heat-labile glycoprotein from commercially available gonadotrophin preparations. The factor strongly inhibited LH binding to Candida membranes, but not to sheep or pig luteal LH receptors. This material had a molecular weight of 16,000-21,000 daltons, bound strongly to CM-Sepharose at physiological pH, and could be resolved completely from hCG and from epidermal growth factor-like factors present in commercial gonadotrophin preparations. Its activity was not attenuated by a range of inhibitors specific for the four major classes of proteolytic enzymes, nor did it inhibit hormone binding by causing degradation of 125I-labelled hLH or hCG tracers. Factors which inhibited hLH binding to Candida membranes were also present in partially purified human urinary and equine serum gonadotrophin preparations and in placental extracts, but were not detected in highly purified CG of hLH preparations. The properties of this factor were similar to those described for beta-core protein, a cleavage product of the beta subunit of hCG which is a contaminant of commercial gonadotrophin preparations. Highly purified beta-core protein inhibited 125I-labelled hLH binding to Candida membranes, but not to sheep luteal binding sites. Preparations of hCG depleted of inhibitor activity could stimulate adenylate cyclase activity in Candida membranes almost five fold. In contrast, partially purified inhibitor preparations strongly inhibited basal adenylate cyclase activity (to 18% of control levels). These observations suggest that endogenous LH-like factors, perhaps similar to beta-core proteins of hCG, may play a role in the regulation of morphogenesis in Candida species.