We have investigated the effect of propolis (CB Propolis) on the growth of human histiolytic lymphoma U937 cells. We found that propolis strongly inhibited the growth of the cells and macromolecular synthesis in a dose- and time-dependent manner by apoptosis. Propolis at 0.015—0.5 μl/ml showed antitumor activity with an IC₅₀ of 0.18 μl/ml for 3 d. It also inhibits DNA, RNA and protein synthesis with an IC₅₀ of 0.08, 0.17 and 4.3 μl/ml, respectively. The inhibitory effect on DNA synthesis was partially irreversible. Moreover, an apoptotic DNA ladder and chromatin condensation were observed in the same concentration range in which cell growth was inhibited. The caspase inhibitor, Z-Asp-CH₂-DCB, prevented DNA fragmentation. These results suggest that the antitumor activity of propolis occurs through the induction of apoptosis. Propolis may be useful as a cancer chemopreventive and chemotherapeutic agent.