This study demonstrates, by freeze fracture, clustering of intramembranous particles caused by cryoprotectant treatment of intact unfixed mouse lymphoid cells. Both T and B cells react in a similar fashion, while similar clustering of particles is not observed in some other cell types. The intramembranous particles can be aggregated by incubating unfixed cells in glycerol or dimethylsulfoxide (DMSO) before freezing. The aggregation phenomenon is dependent on the length of time of exposure and the concentration of the cryoprotectants. Further, the cells remain viable and the cryoprotectant-induced clustering is completely reversible. Prefixation of glycerol-treated cells with glutaraldehyde prevents the formation of these particle clusters, and unfixed nonglycerinated cells show no clusters. Thin sections of cells exposed to glycerol or DMSO without previous fixation exhibit bizarre membrane alterations and numerous other degenerative changes. These observations stress the importance of prefixation of lymphoid cells before exposure to glycerol or DMSO, as well as indicate that the membrane characteristics of certain cell types may be probed by glycerol treatment of unfixed cells.