This report is concerned with histological and electronmicroscopic studies on a spontaneous mammary carcinoma of AKR mice which are known to be high leukemic and low cancer strain. Followings are the conclusions obtained. 1) The spontaneous mammary carcinoma was type B adenocarcinoma according to Dunn's classification. The histology was similar to those frequently observed in mammary carcinoma of high cancer strain of mice such as C(3)H, RIII, and DBA mice. Electronmicroscopic studies revealed many type A virus particles around the cytoplasmic vacuoles or free in the cytoplasm, and numerous type B virus particles In the acinar spaces. In addition, a few type B virus particles were observed in the cytoplasmic vacuoles and in the intercellular spaces. Morphology, distribution, and formation process of these virus particles were similar to those observed in mammary carcinoma of high cancer strain of mice and some of low cancer strain of mice. These results indicate that there exist "milk factor" in low cancer strain of mice as well as in high cancer strain of mice and that the factor can induce mammary carcinoma even in low cancer strain of mice when hormonal and other environmental factors are abequate. 2) Serial electronmicroscopic observations of the original and serially transplanted mammary carcinoma of AKR mice disclosed a gradual decrease in numbers and only a few immature type B virus particles were observed after the 21st generation. These findings were ascribed to the sarcomatous transformation and loss of acinar spaces observed during the serial transplantation of the original mammary carcinoma.. 3) The original mammary carcinoma of AKR mice harbored numerous type A and type B virus particles. Type C particles, however, were hardly observed. In tissue culture cells derived from the tumor, in contrast, numerous type C virus particles were observed and type B virus particles were hardly found. These findings were considered to suggest that there is an interference between type B and type C virus particles in AKR mice.