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SUMMARY: The enhanced pathogenicity of mouse hepatitis virus (MHV 1) in mice infected with Eperythrozöon coccoides is shown to result from the increased probability of virus particles actively infecting cells in the presence of the blood parasite. The ratio (α) of the probability that a virus particle will infect a cell in the absence of E. coccoides to the probability of its doing so in the presence of E. coccoides has been estimated by two independent methods. One method depends upon the increased infectivity of dilute suspensions of MHV 1 for mice pretreated with E. coccoides and gives values for α varying between 0·34 and 0·77, with a mean of 0·5. The other method, in which relatively large doses of MHV 1 are used, depends upon estimating the slope of the regression line of survival time on time of infection with E. coccoides, and yields a value for α of 0·49, with 0·95 probability limits of 0·37 and 0·61. The enhancing effect of E. coccoides on the pathogenicity of MHV 1 is attributed to the parasite increasing the ratio of active to latent infections of cells by the virus.