The effect of extracellular matrix components on cell patterning was studied in cultures of 7-day chick embryo dorsal dermal cells. A scale of ten stages based on cell density, distribution, and patterning has been defined. Starting from a seeding density of 3.5 X 10(5) cells per dish (diameter 35 mm) in 1.7-1.8 ml of medium supplemented with 5% fetal calf serum, cultures reached stage 8 in 7 days. When cells were cultured on a substrate of native bovine type I collagen, their patterning was retarded by 3 to 4 stages. A substrate of human fibronectin had no effect on the rate of cell patterning, when compared with a plastic substrate. However, when fibronectin was adsorbed on collagen-coated dishes, the retarding effect of collagen was suppressed, and a 'normal' rate of cell patterning was restored. When fibronectin was locally adsorbed on plastic or collagen substrates, so as to offer a heterogeneous substrate to the cells, the border between fibronectin and plastic or between fibronectin and collagen was perceived by the cells as a borderline along which they tended to align.