The interaction between the genetically deficient hematopoietic stem cells of W/Wv mice and the stimulation produced by bacterial endotoxin in irradiated and nonirradiated mice was studied. Effects of 25 μg S. typhosa endotoxin injected 1 day before irradiation proved to be similar to effects in +/+ normal mice in several respects. After 300 rads survival was increased from 23% to 77%, recovery of hemoglobin concentration began on day 13 rather than day 17 and granulocyte concentrations were maintained at approximately 1500 per mm3 compared to a drop to <tex-math>$350\ \text{per}\ {\rm mm}^{3}$</tex-math> on day 17 in those given no endotoxin. Erythrocytic spleen colonies were negligible whether or not endotoxin was given. Endotoxin induced granulocytic colonies in significant numbers as early as 5 days after 300 rads, but their peak number (on day 7) was comparable to that seen 4 days after 700 rads in endotoxin treated +/+ mice. Spleen and femur cells of nonirradiated W/Wv mice given nine daily endotoxin injections produced very few erythrocytic colonies on transplantation into normal irradiated recipients, but a fairly substantial number of granulocytic colonies. This number was, nevertheless, far below that for spleen or femur cells of +/+ mice. When W/Wv mice were used as recipients instead of irradiated +/+ mice cells from both normal and W/Wv failed to produce granulocytic colonies. We conclude that endotoxin acts as a stem cell and granulopoietic stimulus in W/Wv mice as it does in +/+ mice, the cellular response to this stimulus being sluggish and subnormal as it is with other stimuli in W/Wv mice.

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