Kaposi's sarcoma, a sexually dimorphic disease inflicting high mortality in AIDS, remains at present without effective treatment. A recent report (Nature 375:64, 1995) showed that the placental glycoprotein hormone, human chorionic gonadotropin (HCG), and surprisingly its beta subunit, inhibit tumorigenicity and metastasis of Kaposi's sarcoma cells in mice xenografts. The anti-KS efficacy of a commercial HCG was subsequently demonstrated in clinical trials. Experimental data presented herein confirm that commercial HCG preparations (known to be about 25% pure) display significant inhibitory action in a dose-dependent manner. However, pure and biologically active HCG has no effect on Kaposi's sarcoma growth in culture. In fact, incubation of Kaposi's sarcoma cells with either one of four different well characterized preparations of pure HCG dimer or any of its two subunits did not alter cellular proliferation suggesting that a contaminant (or degradation product) may be the active agent. Commercial HCG preparations were subfractionated based on molecular size and each fraction was tested with respect to inhibition of KS cell growth, HCG radioreceptor binding and steroidogenic bioactivity. Results demonstrate that the anti-KS activity resides among low molecular weight components, and not in bona fide (macromolecular) HCG. Our study indicates that HCG activity and anti-KS action are separable. Interestingly, the active components in the crude HCG markedly down-regulate AP-1, a complex of transcription factors of the immediate-early response genes associated with cell growth. We conclude that, as yet unidentified molecules, present in the commercial HCG preparations, are responsible for the growth inhibitory effects presumably via the AP-1 signalling pathway.