The data are based on a total of 152 infected mice of inbred stock designated as A-, C-, C3H-, and dba-strains. Males and females compared in this study were of uniform genetic constitution, age, and weight. Results for three strains of T. cruzi are recorded. The "Reichenow" strain (R-strain), originally isolated from a patient in Brazil, was carried exclusively in mice by Dr. E. Reichenow in Hamburg from 1926-1936, by Dr. C. A. Hoare at the Wellcome Laboratories in London since 1936, and in our laboratory since February 1947. This strain is extremely virulent, and usually kills mice of all age groups within three weeks. A culture of the considerably less virulent "Brazil" strain (B-strain) was obtained from Miss E. M. Johnson at the National Institute of Health, Bethesda, Md. The B-strain, derived from a patient in Brazil, was maintained at the National Institute of Health both in vitro and in rats for about six years. Since December 1945, we have kept one sub-line of this strain in culture and another in mice. The past history of the "Culbertson" strain (C-strain), which is relatively avirulent for mice, is incomplete. In August 1936 Dr. J. T. Culbertson brought the material from the London School of Hygiene and Tropical Medicine to New York in an infected bug, Rhodnius prolixus, and propagated the trypanosomes for several years in albino rats (Culbertson and Kessler, 1942). A culture of the C-strain was made available to us in December 1946 by the National Institute of Health. Infectious dosage was uniform in each experimental series. Infected blood (Rstrain) withdrawn from the heart of a mouse at the height of infection was diluted with 5 volumes of sterile physiological saline, and test animals were inoculated subcutaneously with 0.1 cc of this preparation, i.e., about one-half million trypanosomes. T. cruzi from culture (Band C-strains) was injected intraperitoneally in 0.5 cc amounts containing from 30-40 million crithidial forms of the parasite. The precaution of measured inocula was held necessary in this study, although Mazzotti (1940), working with a relatively wide range of dosage (800 to 8000 T. cruzi), found no corresponding variation in the severity of Chagas' disease in mice. The data for blood-populations of T. cruzi are based on thick smears of peripheral blood taken from the tail-tips of infected mice at regular intervals. These preparations were made as uniform as possible. While counts made on thick smears cannot be regarded as absolute numerical values, errors resulting from variations in thickness would tend to be distributed evenly among males and females.