The relationship between plasminogen activator levels and the expression of the transformed phenotype was studied in a 5-bromodeoxyuridine (BrdUrd) dependent mutant of Syrian hamster melanoma cells. In terms of cell morphology and cellular interactions, the BrdUrd dependent cells resemble transformed cells when grown in the presence of BrdUrd but resemble untransformed cells when grown in the absence of BrdUrd. It was found that the BrdUrd dependent cells release significant levels of plasminogen activator only when cultured in the absence of BrdUrd. In the presence of BrdUrd, the release of plasminogen activator by the dependent cells is suppressed, and the decreased level of plasminogen activator released in the presence of BrdUrd seems to be due to the decreased production of active enzyme. Growth tests revealed that the BrdUrd dependent cells, when attached to a substrate, required BrdUrd in order to attain high densities. Furthermore, the cells are able to grow well in soft agar only in the presence of BrdUrd. These results suggest that the production and release of high levels of plasminogen activator are not related (either as cause or effect) to the expression of the transformed phenotype in the BrdUrd dependent cells. The effect of dog serum (as a plasminogen source) on the BrdUrd dependent cells also was tested. It was found that cells cultured in medium containing dog serum exhibit a morphological alteration, but only in the absence of brdUrd. The morphological response of the cells to dog serum resembles that previously observed with virus-transformed cells. In the BrdUrd dependent cells, the morphological response to dog serum appears correlated with the release of plasminogen activator but separated from other transformed characteristics.