Epidemiological studies in China provide reason to suspect that a rich garlic content in the diet might reduce the proliferation of tumors in humans. We conducted experiments on human tumor cell lines and determined the influence of a garlic powder preparation, a garlic extract (reported as 8-10% L(+)-alliin enriched), and a combination thereof, on cellular proliferation in cell cultures, employing the widely used indirect neutral red procedure. Garlic powder failed to inhibit the growth of human hepatoma HepG2 or human colorectal carcinoma Caco2 cells at concentrations of up to 1000 micrograms/ml. Garlic extract, in which the alliin content was highly enriched was also unable to inhibit the growth of these cells. However, when the garlic extract was supplemented with garlic powder (to 10% final concentration) there was a concentration-dependent clear inhibition of tumor cell growth (IC50 values of 330 micrograms/ml for HepG2 and 480 micrograms/ml for Caco-2 cells). The growth of the human lymphatic leukemia cell line CCRF CEM was significantly inhibited in a dose-dependent manner by both garlic powder and garlic extract at concentrations as low as 30 micrograms/ml. However, no potentiation of this effect occurred upon mixing of the two preparations. Our results suggest that the antiproliferative effects of garlic may be due to breakdown products of alliin, such as allicin or polysulfides, rather than alliin itself, since the addition of an alliinase system (garlic powder) to an alliin enriched preparation without alliinase (garlic extract) potentiated the effects observed with the two preparations alone.