The morphology of collagen fibrils at various times during formation in vitro was quantitatively examined by negative staining and by scanning electron microscopy. The presence of a small dermatan sulfate proteoglycan from bovine tendon (5 micrograms proteoglycan/100 micrograms collagen) resulted in collagen fibrils that were significantly thinner in width at all times by both methodologies. The rate of fibril diameter increase was also retarded by the small proteoglycans, suggesting that they inhibited the lateral aggregation of forming collagen fibrils. Large proteoglycans from cartilage did not produce this effect.