The effects of aflatoxin B1 (AFB1) were studied using the HepG2 cell line. Cytotoxicity, apoptosis and p53 expression were assessed after exposure to different concentrations of AFB1 (0–100 μm) and its two types of degradation products, namely the mixtures of photodegradation products in water (Pw) and the mixtures of photodegradation products in peanut oil (Po) for different time periods (0, 24, and 48 h). After exposure of the HepG2 cells to these compounds for different times and concentrations, the cytotoxicity of Pw and Po decreased approximately 40 and 100% compared with AFB1, respectively. The expression of p53 protein decreased significantly in AFB1-exposed cells, decreased slightly in Pw-treated cells and did not decrease compared to the untreated cells. The results of the in vitro cytotoxicity assay indicate that Pw is less toxic than AFB1, and Po has almost no toxicity, which can be explained by the differences in the chemical nature of the various kinds of the test compounds. Copyright © 2011 John Wiley & Sons, Ltd.