Formation of foci of Rous cells by B-RSV (RAVI), in medium containing agarose and tryptose phosphate broth (TPB), is increased 2- to 10-fold (or more, in crowded cultures) by addition of sulfated polyanions (dextran sulfate [DS], heparin), at the post-infection stage, lowering the concentration of Ca++ and Mg++ in the medium, or raising to 41° the temperature of incubation. These nonspecific factors may all favor cell transformation by reducing cell adhesion. DS and high temperature also enhance focus formation by B-RSV (RAV2) but not by PR-RSV (C) or SR-RSV (D). Omission of TPB from the medium reduces 10- to 100-fold the number of foci produced by B-RSV and 5- to 10-fold that produced by PR-RSV or SR-RSV; and TPB is also required for enhancement by other nonspecific factors of focus formation by B-RSV. Hence, it must act differently from these factors.