Band patterns exhibited under electron microscope by native collagen fibrils fixed with glutaraldehyde (2.5%-5% GA diluted in 0.1M phosphate buffer, pH 7.4) and negatively stained with phosphotungstic acid (1% PTA diluted in the same buffer) were digitized to form both bandings and microdensitometric traces. Collaterally, computer-drawn band patterns and traces were yielded on the basis of the "quarter stagger" model and primary structure of alpha 1(I) and alpha 2(I) tropocollagen chains and by selecting options related to specific collagen-GA interactions. Comparisons between actual and simulated patterns suggest that lysines and hydroxylysines should react with GA residues in a 1:3 ratio, while GA-reactivity of histidyl and tyrosyl residues seem to be excluded. On the other hand, an improvement of simulations was achieved by also selecting hydroxyprolines (in addition to lysines and hydroxylysines), which seemed to react with GA in a 1:1 ratio. Considering the bifunctionality of GA, it is suggested that during fixation, heteropolymers form, composed of GA-hexamers bonded to couples of lysyl and/or hydroxylysyl residues. The hypothesis is advanced of an additional formation of GA-dimers, each bonded to two hydroxyprolines.