Steady state fluorescence polarization studies were carried out on membranes of polyoma-virus-transformed, revertants, re-revertants, and normal golden hamster cells. Four clones of revertant cells which exhibit different levels of reversion were isolated. The degree of reversion in these revertant clones was characterized in vitro by their contact inhibition behavior and in vivo by their tumorigenicity to hamsters. A good correlation between these two criteria was observed. Re-revertant cells were obtained from that revertant clone which exhibited the highest degree of reversion (similar to normal cells) and which could still produce tumors in hamsters. Re-revertants resembled the polyoma-virus-transformed cells in both contact inhibition and tumorigenicity. Fluorescence polarization values of 1,6-diphenyl-1,3,5-hexatriene-labelled cells correlated well with cell tumorigenicity: normal and revertant cells which were not malignant exhibited distinctly lower fluorescence polarization values than those of the tumorigenic transformed and re-revertant cells. Intermediary revertant clones exhibited fluorescence polarization values in between those of normal and transformed cells, yet indistinguishable within experimental uncertainty.