Low doses of radiation reduce the rate and magnitude of fluorescent probe interaction with spleen cells. The uptake of 8-anilino-naphthalene sulfonic acid (ANS), but not diphenyl-hexatriene (DPH), is reduced in a dose-dependent fashion in cells that receive 25-100 rads. The effects of irradiation are most evident in a "medium" fluorescent subpopulation with no effect observed for a "dim" subpopulation. Splenic lymphocytes enriched for T cells show lower uptake of ANS, compared with a B-cell-enriched population. In addition, the normal decrease in ANS fluorescence polarization that occurs during the initial 10 minutes of probe interaction is attenuated in irradiated cells. These findings are consistent with the notion that low doses of radiation limit the penetration of ANS, but not DPH, into a less restrictive microenvironment within the plasma membranes of T, but not B, cells.