Fusion of noninvasive, nonmetastatic BW5147 T-lymphoma cells with normal T-lymphocytes usually resulted in highly invasive and metastatic T-cell hybridomas, apparently due to properties derived from the normal T-cell. Occasionally hybrids arose that were non- or low invasive, probably by loss of relevant genes upon chromosome segregation, since these cells contained much less DNA than highly invasive hybrids. The metastatic potential of 20 representative T-cell hybridomas was tested by tail vein injection in syngeneic mice and cells were found to be either nonmetastatic (NM), low metastatic (LM), or high metastatic (HM). NM hybrids were tumorigenic but did not form metastases and HM hybridomas caused wide-spread metastasis. LM cells formed metastases in a limited number of mice and predominantly in lymphoid tissues. In hepatocyte cultures, NM cell lines were found to be the least invasive, HM cells the most, whereas LM hybrids exhibited intermediate levels. Invasiveness was not only measured in rat hepatocyte cultures but also in rat embryo fibroblast monolayers, and the relative invasive capacity in both model systems correlated well. Pertussis toxin inhibited invasion in both systems to 20-30% of control values. This suggests that the mechanisms of invasion into hepatocyte and fibroblast cultures are at least partially similar and that the fibroblast invasion assay is a relevant model to study aspects of lymphoma metastasis. We conclude that invasive potential is a prerequisite for T-cell hybridomas to colonize tissues from the bloodstream and that a minimum level of invasiveness is necessary for extensive and wide-spread metastasis formation.