A bidirectional selection experiment for increased and decreased somatic cell counts (SCC) in milk was conducted with two selected lines (high line or HSCC, low line or LSCC) and one control line (CSCC) of mice. Distribution of milk SCC in mice was 4 to 5 times the distribution in Holsteins. The shape of the lactation curve of mice was similar to that of Holsteins with maximum yield at day 7 (2.06 gms). The phenotypic regression of lactation milk yield on lactation milk SCC (-0.276) was significant (P < .05) and indicated that dams with higher SCC produce less milk. Small phenotypic correlations of milk SCC with blood SCC (-0.09) and percentage phagocytic cells (-0.06) were found, indicating that these traits are unrelated. A small but significant (P < .05) negative correlation (-0.14), was found between response to endotoxin challenge and milk SCC indicating that dams with inherently higher milk SCC responded less to endotoxin challenge than dams with lower milk SCC. No major pathogens which cause mastitis in cattle were detected in milk of mice. In addition, the bacteria identified (Bacillus sp., Corynebacterium sp. etc) did not cause any serious infections and/or increase in milk SCC in mice. Selection for high and low milk SCC produced a symmetrical response in the two selected lines (HSCC and LSCC), such that after 7 generations of selection, the two lines differed by more than 500,000 cells/ml of milk. A small negative genetic regression ( -0.162) of milk yield on milk SCC suggested a small correlated response in yield opposite in direction from the direct response for milk SCC. Small negative genetic regressions of blood SCC and percentage phagocytic cells on milk SCC ( -0.087 and -3.492) suggested that these three traits are genetically independent. Selection on milk SCC did not result in change either in total leukocytic cells per ml of blood or in percentage phagocytic leukocytes in blood. A negative genetic regression of response to challenge on milk SCC (-3.201) was found suggesting that selection for low milk SCC results in an increase of the ability of the individual mouse to elevate milk SCC after an injection with E. coli endotoxin. However, more data are needed to confirm this conclusion. Phenotypic correlations between milk SCC and several measures of fitness and genetic regressions of these measures on milk SCC were negligible. / Ph. D.