The effects of intravenous (iv) inoculation of rat prostate R3327-MatLyLu tumor cells in syngenic Copenhagen rats have been investigated both qualitatively and quantitatively in order to establish an in vivo clonogenic cell assay. Intravenous injection of tumor cells resulted in formation of tumor nodules only in the lungs. A linear relationship existed between the number of tumor cells injected and the number of measured lung nodules. The distribution patterns of iv injected radiolabelled tumor cells and of non-labelled tumor cells injected in both arterial and venous blood vessels confirmed the major role of the lungs as recipient organ for injected tumor cells. Although a similar distribution pattern was observed after iv injection with radiolabelled fibroblasts, no lung nodules were observed even one year after injection. The sensitivity for measuring drug treatment effects by this in vivo clonogenic cell assay has been compared with that of the in vitro clonogenic cell assay. Information obtained using both in vitro and in vivo clonogenic cell assay will provide a better understanding of efficacy of treatment modalities.