Rat 13762NF mammary adenocarcinoma cloned cell lines were assayed at different in vitro passage numbers and compared for their abilities to form 'spontaneous' metastases by subcutaneous injection of cells and 'experimental' metastases by intravenous injection of cells. Tumor cell clones were established from locally growing tumor and spontaneous lung metastases, and these clones were found to possess heterogeneous metastatic potentials in both metastasis assays. The rank order of clonal metastatic potentials based on either the average number of lung tumor colonies or the average total lung tumor volume was generally equivalent for 'spontaneous' and 'experimental' metastases, but some differences were noted. Ranking of 'spontaneous' metastasis by average total lung tumor volumes more closely resembled the rank order of 'experimental' metastasis than by the average number of spontaneous metastases. The results demonstrated that in the 13762NF mammary adenocarcinoma system (i) there is heterogeneity in tumor cell clonal metastatic potential using either 'spontaneous' or 'experimental' assays; (ii) these two assay methods yield generally the same rank order of metastatic potential; (iii) the metastatic potential of each of the tumor cell clones drifts with time (passage number) in cell culture, and (iv) ranking by average tumor burden calculated from total lung tumor volumes may yield a better estimate of metastatic potential than ranking by the average number of lung tumor colonies.