Previous studies demonstrated that growth in DBA/2 mice of MDW4, a wheat germ agglutinin-resistant (WGA^r) mutant of the highly metastatic MDAY-D2 DBA/2 mouse tumor, led to the emergence of WGA-sensitive (WGA^s) revertants having higher ploidy levels at the site of inoculation as well as at distant visceral metastases. The results implied that MDW4 was nonmetastatic but progressed to become metastatic in vivo only after a cellular change took place which was accompanied by extinction of the WGA^r phenotype and acquisition of a higher number of chromosomes. Results presented here provide strong and direct evidence for the underlying mechanism being spontaneous cell fusion in vivo between the MDW4 (WGA^r) tumor cells and normal host cells, at least some of which are of bone marrow origin. Thus, growth of the H-2^d MDW4 tumor cells in (C3H × DBA/2)F₁ (H-2^k × H-2^d) or (C57BL/6 × DBA/2)F₁ (H-2^b × H-2^d) mice led to the appearance of WGA^s revertants bearing the H-2^k or H-2^b major histocompatibility complex antigens associated with the C3H or C57BL/6 parental strains, respectively. Similarly, WGA^s revertants of MDW4 were found to express H-2^k antigens after growth in CBA/HT6T6 (H-2^k)→DBA/2 bone marrow radiation chimeras. Attempts to mimic the in vivo hybridization process were successful in that in vitro somatic cell fusion between an ouabain-resistant (Oua^R), 6-thioguanine-resistant (Thg^r) derivative of the MDW4 mutant and either normal bone marrow or spleen cells resulted in loss of the WGA^r phenotype in the hybrids (thus showing its recessive character) and increased malignant properties in vivo. An analysis of spontaneous frequencies of re-expression of various drug resistance genetic markers in several hybrid metastatic cells was also consistent with chromosome segregation of the sensitive alleles. The results show that tumor progression and the emergence of metastatic cell variants could arise as a consequence of tumor × host cell fusion followed by chromosome segregation. We also discuss the possibility that this type of event may normally be a very rare one during the growth of tumors, the frequency of which can be artificially amplified by the use of certain classes of lectin-resistant mutants carrying particular cell surface alterations.