Spontaneous primary mammary tumors of C3H-Avy mice differ in metastatic colonization potential, some producing many lung deposits (high-colonization potential) and others producing few or none (low-colonization potential) after iv inoculation of cells. The degree of metastasis from undisturbed neoplasms also varies from tumor to tumor. This study examined whether these differences between tumors could be accounted for by differences in clonogenic or stem cell content. Tests for clonogenic cells were: growth in 0.3% agarose and limiting dilution assays. Mammary tumor cells of known colonization potential were inoculated iv at serially reduced doses, and the relationship between number of cells injected and number of lung deposits formed was determined. Parallel in vitro dose-response assays in 0.3% agarose were performed with the use of cells from the same primary tumors. Colony-forming efficiency in 0.3% agarose cultures varied between individual primary mammary tumors and was positively associated with experimental metastatic potential, suggesting that the stem or clonogenic cell content of primary tumors is one of the important determinants of the metastatic phenotype.