Rotation-mediated aggregation of human skin fibroblasts has been studied and the patterns of aggregation compared between cultures obtained from 9 patients with Duchenne muscular dystrophy and from 10 normal controls. The rate of aggregation was dependent on the growth state of the cells, with growing cells aggregating more rapidly than growth-arrested cells. There was considerable variation between individuals in the rate at which cells aggregated but no differences were detected in the aggregation of normal and DMD cells measured by this method. The monovalent cation ionophore monensin, which inhibits the transport of cellular proteins to the extracellular medium, reduced aggregation of all cells. The data suggest that after initial cell-cell contact continued aggregation is dependent on the secretion of materials to the cell surface. The aggregation of normal and DMD cells was affected similarly by this treatment.