An individual's susceptibility to cancer is assessed based on the individual's cellular response to mutagenic agents such as radiation. The level of a growth-suppressing marker is measured before and after the individual's cells are exposed to the mutagenic agent. The individual's susceptibility to cancer as a result of the mutagenic agent is correlated with the degree to which the growth-suppressing marker is induced by exposure to the agent. A method is also disclosed for assessing cancer prophylaxis effects of compounds, such as vitamins or food extracts, in individuals. Cells from an individual are incubated with at least one compound in vitro, or the compound is directly administered to the individual, after which some of the incubated cells, as well as non-incubated cells, are exposed to a mutagenic agent such as ionizing radiation. The level of the growth-suppressing marker in the cells incubated with the compound and exposed to the mutagenic agent is then compared with the level in the non-incubated cells exposed to the agent. The cancer prophylaxis effects of the compound are correlated with a higher level of the marker in the incubated cells.