To follow the cellular progeny of the multiple-drug-marked benign murine tumor cell line MDW4 during its progression in vivo toward metastatic spread in DBA/2 mice, the following parameters were analyzed: retention of the drug-resistant markers ouabain resistance (OuaR) and thioguanine resistance (ThgR), lectin-resistance pattern (WGAR), and the karyotype of cell populations (and clones derived from these cells) removed at intervals from the solid tumor growing at the site of inoculation, as well as distant metastatic nodules. It was determined that the initially homogeneous inoculum composed of OuaR, ThgR, and WGAR hypotetraploid cells (mode: 68 +/- 2 chromosomes) was gradually overgrown and replaced by a new population of cells that were either OuaR or ouabain-sensitive but that became thioguanine-and lectin-sensitive and hyperploid (mode: 95 +/- 5). Regardless of the composition of the individual drug marker combinations, only cells with high chromosome contents were found to be able to disseminate to distant visceral organs and to rapidly produce metastases upon sc or iv reinjection. The presence of the same number of metacentric chromosomes in metastatic cells as in MDW4 and the coextinction of two recessive drug-resistant markers (WGAR and ThgR) suggested that cells endowed with invasive-metastatic potential represent the product of spontaneous somatic hybridization between the original nonmetastatic MDW4 cells and normal host cells of unknown origin. Such a fusion was followed by more or less extensive chromosome segregation that accounts for the karyotype mosaicism and the occasional drug marker heterogeneity identified in cell populations of metastatic nodules.