The present work aimed to study the mutagenic effect of aflatoxin B1 on human chromosomes. The experiments showed that aflatoxin B1 is a strong chromosome damaging agent. The treated cells showed a high rate of aberrations mainly breaks and interchanges. Using Giemsa banding technique, the study showed that the distribution of breakage points on individual chromosomes was significantly non-random. The study of sister chromatid exchanges (SCEs) indicated that the high incidence of breakage rate was paralled by an increased SCE rate. Some chromosomes were very sensitive to the mutagenic effect of aflatoxin B1, while other chromosomes were very resistant. The present data provides an additional consideration in assessing the risks of exposure to this agent.