Characterization of a rabbit model system for the study of cell cycle effects of myelotoxic agents in normal bone marrow is described. Cell cycle phase distributions are obtained by computer analysis of flow cytometric single cell DNA histograms. Comparison of marrow aspirates with marrow samples from sacrificed animals indicates that dilution of aspirates with peripheral blood is not significant. Aspiration of marrow from one bone does not affect the cell cycle distribution of unsampled bones. Hence, sequential aspirates of different bones in a single animal may be used as representative samples for further study of effects of myelotoxins on marrow proliferation and differentiation.