Resistance to P. berghei in randomly bred CD-1 mice is influenced by the number of animals housed per cage with individually housed mice showing the greatest resistance. One additional randomly bred and 3 inbred strains were housed either 1 or 5 per cage and no differences in resistance were noted, although the previously described differences in resistance were again observed in CD-1 mice. In CD-1 mice, no differences were noted in plasma interferon levels between groupand individually housed animals. The mortality rate of mice infected with Plasmodium berghei has been related to the number of animals housed per cage, with an increasing rate of death noted as larger numbers of mice were housed together (Plaut et al., 1969). Within the limits tested, cage size was not an influence on resistance and therefore the effect was related to population size rather than population density. Grouped mice separated by screening died at the same rate as individually housed animals, suggesting a role of physical contact in the higher mortality rates observed in group-housed mice. If housing conditions were reversed immediately following infection, differences in mortality rate were primarily dependent on the housing condition following, rather than prior to, inoculation with P. berghei. In subsequent work (Plaut, Friedman, and Grota, 1971), it was shown that the effect of differential housing upon resistance was abolished by splenectomy. In all of these studies, a randomly bred strain, CD-1 mice from Charles River Breeding Laboratories, was used. The influence of housing upon resistance to P. berghei in four additional strains of mice is the basis for this report. Received for publication 29 March 1973. * The authors wish to acknowledge support from U. S. Public Health grants MH-16741 and K3-MH-18,542 awarded by the National Institute of Mental Health and AI-10217 from the NIAID and the technical assistance of Darbbie Mahany, Linda Cowles, and Ann Murrer. t Current address: Division of Child and Adolescent Services, Department of Psychiatry, University of Maryland School of Medicine, Baltimore, Maryland 21201. MATERIALS AND METHODS Animals for mortality studies CD-1 mice were obtained from the Charles River Breeding Laboratories (Wilmington, Mass.); a second randomly bred Swiss Webster (MLM-1) strain was obtained from Western New York Animal Resources (Ontario, New York); C57B1/J, BALB/c, and A/J inbred strains were obtained from the Jackson Laboratory (Bar Harbor, Maine). Animals were received from suppliers at 3 to 8 weeks of age, and housed individually or 5 per cage for 2 to 6 weeks. At the time of inoculation, the strains of differentially housed animals ranged from 9 to 12 weeks of age. Deaths were recorded daily, and the experiment terminated when no deaths occurred for 5 consecutive days and the surviving animals appeared well. All groups of individually or group-housed mice contained 25 animals, except the A/J strain which contained 15 mice. Laboratory conditions, origin of the KBG-173 P. berghei strain, and method of inoculation with approximately 1 X 106 parasitized red blood cells have been previously described (Plaut et al., 1969). Comparison of rates of death were determined by the Mann-Whitney U test (Siegel, 1956). Animals for interferon study Female 6-week-old mice were obtained from the suppliers mentioned above, and half of each group housed individually or 3 per cage. [Previous work (Plaut et al., 1969) indicated that mice housed 3 per cage would be expected to differ in mortality rate from animals housed individually.] At approximately 3 months of age, half of the total group, 120 mice of each strain, were inoculated with P. berghei. For each sampling time, 3 groupand 3 individually housed mice of each strain were killed by decapitation. Blood from each strain under the two housing conditions was pooled in heparinized tubes and centrifuged for 10 min at 2,000 rpm and the plasma immediately