ABSTRACTA study was made of the effects of cytochalasin B on (a) specific sorting of reaggregating cells; (b) redistribution of cell types after treatment of preformed aggregates; and (c) the ability of aggregates of one cell type to incorporate and sort cells of another type. Freshly disaggregated neural retina and heart cells were cultured on a gyratory shaker at 70 rev/min and the aggregates formed analysed for sorting of cell types.Cytochalasin B disrupted the sorting of forming aggregates at concentrations of 1 μg/ml and greater. The distribution of cell types in aggregates that were treated with cytochalasin after 24 h of culture was more random than the control.Untreated cultures of retinal aggregates and heart cell suspension resulted in pure retinal and pure heart aggregates, but with more than 50% mixed and sorted aggregates. Cytochalasin B treatment resulted in fewer mixed aggregates and a higher proportion of pure retina and pure heart aggregates.