Elveback, L (Dept. Medical Statistics, Mayo Clinic, Rochester, Minnesota 55901), E. Ackerman, L Gatewood and J. P. Fox. Stochastic two-agent epidemic simulation models for a community of families. Amer J Epidem 93: 267–280, 1971.—Two stochastic, discrete-time-interval simulation epidemic models which allow for competition and interference between a wild enteric virus and the live poliovirus vaccine are presented. The first is for a community of families in which each individual belongs to several mixing groups which depend on his age and which have different contact rates. The wild virus is taken to be one of the coxsackie group and the vaccine as the Sabin poliovirus vaccine. The latent period is half of one interval and the infectivity periods are discrete random variables. Interference operates for the first three intervals after infection and prevents infection with the second agent. Results are presented in terms of the distribution of epidemic size as estimated from 100 epidemic trials. The effects of opening and closing of the school, of the placement of the original six cases in the population, and of the percentage of the population vaccinated are explored. In the second model, environmental contamination is introduced as a method of spread, and it is assumed that at any time 50% of the population have enteric virus infections which result in vaccination failure. The wild virus is taken as poliovirus and the vaccine as the monovalent Sabin poliovirus of the same type. It is shown that in such a situation, which is known to exist in some of the developing countries, vaccination with live poliovirus vaccine can reduce the size of a threatening epidemic of poliomyelitis.