The in vitro anticarcinogenic potential of propolis in human lymphocytes was investigated. Blood samples were obtained from ten healthy males, non-smoking volunteers, which were incubated and exposed to increasing concentrations of propolis (0.01, 0.05, 0.1, 0.2, 0.5, 0.7 and 1.0 ml). The mean micronucleus rates were 1.47 +/- 0.38-4.02 +/- 0.64. Mitotic index rates were between 19.45 +/- 2.22 and 0.28 +/- 0.33. The differences between the control and exposed cells were statistically significant (p <= 0.05). We conclude that exposure to different concentrations of propolis cannot produce a carcinogenic effect in peripheral human lymphocytes in vitro. However, increasing micronucleus (MN) rates showed that propolis could have a carcinogenic effect in high concentrations.