We assessed hematologic recovery, body weight, and behavior after serial blood collection in 10- to 14-wk-old C57BL/6 mice. Male and female mice (5 to 11 mice for pilot groups, 23 to 35 mice for full study groups) had either 15%, 20%, or 25% of their estimated total blood volume (TBV) collected once weekly for 6 wk. Except for those of the 25% TBV male pilot group, the weights of all mice recovered or increased from one collection to the next. The behavior of all mice, with the exception of the 25% TBV male pilot group, appeared normal throughout the study. Erythrogram value changes from baseline were analyzed at each weekly blood collection. Recovery was defined as the return of mean hemoglobin values to within 2 SD of mean baseline values. According to this definition, mice in the 15% TBV male group and 15%, 20%, and 25% TBV female groups recovered hematologically. To support the statistical definition of recovery, we compared our data with human anemia categories to assess the clinical relevance of the mouse hemoglobin values. On the basis of these data, we conclude that as much as 25% TBV can be collected once weekly from female mice for 6 wk and as much as 15% TBV can be collected once weekly from male mice for 6 wk without producing weight loss, behavioral changes, or clinically significant anemia.