Single-stranded short-chain RNA fragments, obtained by mild degradation of purified Escherichia coli ribosomal RNA(s) with pancreatic RNase A, exhibit particular biologic activities in vitro and in vivo. In vitro, these RNA fragments are used by DNA-dependent DNA polymerase I as primers to initiate the replication of DNA(s) isolated from rabbit bone marrow and spleen; they are inactive with DNA isolated from several normal tissues and cancerous cells. Administered iv, RNA fragments restore a normal level of circulating leukocytes in rabbits with high doses of cyclophosphamide (CP). Granulocyte/lymphocyte balance, upset by daily CP administration, is also restored during the increase of both types of cells. No toxicity is observed, and numerous repeated doses of RNA fragments show no cumulative effect and do not lead to loss of leukopoietic stimulating activity. Tumor-bearing mice can be protected by RNA fragments against the toxic effect of CP without impeding the anticancer activity of this drug.