Transplanted tumors in rats and mice have been used extensively in radiobiology studies examining the biological basis and therapeutic implications of hypoxia in solid tumors, and in studies evaluating new agents and regimens which may circumvent the radioprotective effects of hypoxic cells. This use of rodent tumors assumes that data obtained using rapidly-growing transplanted tumors arising in inbred rodents can be extrapolated meaningfully to the treatment of primary and/or metastatic lesions in heterozygous humans. The studies reported here examine one facet of this critical assumption by comparing the hypoxic fractions of transplanted rodent tumors with those of human tumor cell lines xenografted into immune deficient mice. No significant differences were found between the hypoxic fractions of the xenografts and those of mouse tumors. This finding eliminates several possible bases for predicting that the oxygenation of human tumors might be systematically different from that of the rodent tumors often used in preclinical radiobiology studies, and supports the hypothesis that human tumors contain hypoxic cells which may be critical in determining their response to therapy.