The distribution of intramembrane particles (IMP) as revealed by freeze-fracture electron microscopy has been analyzed following treatment of mouse L cells and fusion-deficient L cell derivatives with several concentrations of polyethylene glycol (PEG). In cell cultures treated with concentrations of PEG below the critical level for fusion, no aggregation of IMP was observed. When confluent cultures of the parental cells are treated with 50% PEG, greater than 90% of the cells fuse, and cold-induced IMP aggregation is extensive. In contrast, identical treatment of fusion-deficient cell lines shows neither extensive fusion nor IMP redistribution. At higher concentrations of PEG, however, the PEG-resistant cells fuse extensively and IMP aggregation is evident. Thus the decreased ability of the fusion-deficient cells to fuse after treatment with PEG is correlated with the failure of IMP aggregation to occur. A technique for quantifying particle distribution was developed that is practical for the accurate analysis of a large number of micrographs. The variance from the mean number of particles in randomly chosen areas of fixed size was calculated for each cell line at each concentration of PEG. Statistical analysis confirms visual observation of highly aggregated IMP, and allows detection of low levels of aggregation in parental cells that were less extensively fused by exposure to lower concentrations of PEG. When low levels of fusion were induced in fusion-deficient cells, however, no IMP aggregation could be detected.