In vitro "simultaneous processing" was investigated in which fibril formation of collagen and cross-linking occur simultaneously in the presence of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) as a cross-linking reagent. Fibril formation in simultaneous processing was monitored using turbidity. The EDC in simultaneous processing increased T-1/2 (time required for half of the plateau value in turbidity) and decreased the degree of the fibril formation dose dependently. The reduced fibril formation rate (T-1/2 > 60 s) suggests the introduction of intrafibrillar cross-linking during fibril formation. The collagen gels prepared using simultaneous processing had a compressive modulus that was 6-fold higher than that using sequential processing, which is an advantage of simultaneous processing. Atomic force microscopy images acquired under water on the wet gels demonstrated that the simultaneous processing provided a unique double-network structure: intrafibrillarly cross-linked collagen fibrils among which nonfibrous collagens act as interfibrillar cross-linkages.