Two related phenomena take place in dog kidney (DK) cells doubly infected with MP dk −, a conditional lethal strain of herpes simplex virus, and MP dk + sp, a mutant capable of multiplying in DK cells. On one hand, the formation of infectious MP dk − virions is considerably increased (enhancement). On the other, the yield of infectious MP dk + sp is greatly reduced (interference). The relationship between enhancement and interference was studied by incubating DK cells at different temperatures after simultaneous and after staggered infection with the two mutants. The results show that conditions of infection which favor enhancement also favor minimal interference. Conversely, conditions favoring maximal interference do not favor enhancement. The results do not satisfy the hypothesis that MP dk − competes with MP dk + sp for cellular sites or for functional proteins specified only by MP dk + sp virus. The results are in accord with the predictions arising from the hypothesis that interference results from the formation of nonfunctional aggregates consisting of defective MP dk − subunits and potentially functional subunits specified by MP dk + sp.