New fluorescent probes of membrane mobility can be introduced into cell membranes at single points with particles of a membrane mobility agent, A2C. The initial entry of fluorescence from the particle into the cell membrane and the subsequent lateral spread of fluorescence have been observed for cells in suspension. A dramatic difference between the behavior of normal lymphocytes and that of mitogen-transformed and mastocytoma cells is found. Both the initial entry and the spreading of fluorescence are much slower in the transformed and tumor cells than in the normal cells at 18°C. Entry and spread of fluorescence in normal cells become slow enough to be observed only at 12°C or below.