Corneal transparency is dependent on the size and arrangement of collagen fibrils within the corneal stroma. The corneal stroma is composed primarily of collagen type 1 fibrils and two proteoglycans: one with chondroitin/dermatan sulfate side-chains (decorin) and one with keratan sulfate side-chains (lumican). We investigated the effects of the corneal proteoglycans on corneal collagen fibrillogenesis, utilizing an in vitro assay for fibril formation. Collagen was extracted from bovine corneal stromas with 0.1 M acetic acid and monomers purified by NaCl precipitation. Decorin and lumican were extracted from bovine corneal stroma with either 0.7 M NaCl or 4 M guanidine HCl and purified by DEAE and Sepharose CL-4B chromatography. Decorin and lumican from both extracts inhibited the rate of collagen fibrillogenesis and the development of turbidity in fibrillogenesis samples. Furthermore, the core proteins of decorin and lumican were shown to be as effective as the intact proteoglycans in inhibiting fibrillogenesis. The decorin core protein isolated from the 0.7 M NaCl extract was determined to be a 20 kDa fragment which lacks the C-terminal half of the core protein. This fragment was approximately 1/36 as effective in inhibiting fibrillogenesis as intact decorin isolated from guanidine extracts. This suggests that the C-terminal half of the decorin core plays an important role in the interaction of this proteoglycan with collagen. Lumican extracted with 0.7 M NaCl was slightly smaller and was only one-sixth as effective in inhibiting collagen fibril formation as 4 M guanidine extracted lumican. Furthermore reduction and alkylation of lumican core protein abolished the inhibitory activity of the core protein on collagen fibrillogenesis. Electron microscopic examination indicated that fibrils formed in the presence of lumican and lumican core protein were significantly thinner than fibrils formed in the absence of proteoglycans. The results of these studies indicate that in addition to decorin, lumican retards corneal collagen fibrillogenesis and results in the formation of collagen fibrils which are significantly thinner than those formed in the absence of any proteoglycan. The inhibitory activity of lumican or decorin on collagen fibrillogenesis resides in he core proteins of these proteoglycans, not the glycosaminoglycan side chains, and that interaction of the lumican core protein with collagen appears to be dependent on the presence of disulfide bridges within the protein core.