Four strains of mice—Harvard, Schwentker, CFW (Carworth Farms), and C (National Cancer Institute)—have been kept and bred underidentical conditions over the course of a year in order to determine the effect of strain, of litter, and of season upon the incidence of diarrheal disease among the suckling mice.

1. Significant strain differences were limited to CFW mice. A consistently lower percentage of these mice were weaned in comparison to the others.

2. The effect of litter was definite and consistent: In all four strains first litters fared worse than second and third ones, while fourth and fifth litters showed the highest percentage of mice weaned.

3. The effect of season seemed clear cut only in the case of one strain CFW; the highest percentage of mice weaned were observed during the summer months and the lowest during the autumn and early winter.

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