[en] Quantal health effects caused by the combined action of a number of toxic agents are modeled using the information available for each toxicant acting in isolation. Two basic models are used; one assumes no interaction, the other postulates a separable kind of interaction in which each agent contributes an enhancement factor independent of all other agents. These two models provide yardsticks by which to measure synergisms and antagonisms in the interaction between the effects of toxic agents. Equations are given in approximations for small and large values of the risk. (author)