A direct assay of granulocytic repopulating ability.

A direct method of assay of the 'granulocytic repopulating ability' (GRA) of bone marrow in normal or treated mice, which utilizes determinations of total bone marrow cellularity and peroxidase positive cells, is described. Ten days after transplantation of normal femoral marrow, the total number of marrow granulocytes in the recipients is proportional to the marrow dose in the range 0.01 to 0.03 of a femur. Above 0.01 femur the proportion of granulocytes is constant at about 60% so there is a high correlation between total cellularity and total granulocytes. Assay of erythroid repopulating ability can be performed in the same animals. Using the GRA assay, the results for the dose response curve for cyclophosphamide, and the relationship of GRA to erythroid repopulating ability (ERA) and spleen colony forming cells, are similar to those obtained using the endotoxin technique. The simplicity of the GRA technique enhances the feasibility of multiple simultaneous hematopoietic precursor cell assays.