By means of the indirect membrane immunofluorescence test, the distribution and antibody-induced redistribution (patching and capping) of a mammary tumor virus-induced (MLr) and a normal (Thy 1.2) cell-surface antigen were compared on mouse thymocytes and leukemia cells (GRSL2). At 0 degrees C Thy 1.2 fluorescence was ringlike and more intense on GRSL2 cells than on thymocytes, whereas MLr fluorescence on GRLS2 cells at this temperature was patchlike and brighter than Thy 1.2 fluorescence. At 20 or 37 degrees C, capping of Thy 1.2 on both cell types was readily achieved but MLr capping occurred only in a few GRSL2 cells and was less pronounced. However, after addition of the secondary antibodies, MLr capping was markedly increased by gradual cooling of cells to about 17 degrees C. Conversely, after addition of antibodies at 0 degrees C, gradual warming of cells under the fluorescence microscope resulted in extensive capping both of MLr and Thy 1.2 at approximately 13-14 degrees C. Rapid cooling or rapid warming led to almost instantaneous capping. These results may be explained by the occurrence of phase transitions or phase separations in the particular temperature range. Another difference between capping of Thy 1.2 and MLr was that the former caps were small and eventually were endocytosed, whereas the MLr caps were large and were exfoliated from the cells.