Mineralization at collagen fibrils is regulated by glycosaminoglycans (GAG). Alterations in proteoglycan composition during mineralization as well as inhibition of mineralization by GAGs are well documented. Collagen-GAG interactions during desmoid osteogenesis in fetal rat calvariae were investigated ultrastructurally by means of different fixation techniques. Mineralization was restricted to the collagen of the osteoid at the ectocranial side. Beyond the osteoid, one layer containing degenerated cells was found, followed by sheets of healthy osteoblasts with nonmineralized collagen fibrils. These fibrils were ordered in bundles, but were irregularly arranged in the mineralized osteoid. After fixation in glutaraldehy deruthenium red (GA-RR), small RR-positive granules were periodically attached to the fibrils of the nonmineralized collagen. These granules were absent at collagen in the mineralized osteoid. Periodically bound granules (periodicity of 62 nm) could clearly be demonstrated along collagen fibrils by pretreatment with the positively charged protamine sulfate and subsequent fixation in GA-RR in the nonmineralized collagen. In the mineralized osteoid, however, these granules were present, but periodic binding was missing. Heparin pretreatment followed by fixation in GA-RR revealed periodically bound fine strands between collagen fibrils running parallel in the nonmineralized collagen; these threads were absent in the mineralizing osteoid. Restriction of mineralization to osteoid at the mineralization border may be reflected by the observed changes in GAG binding to collagen fibrils within the osteoid of developing fetal calvariae in contrast to binding to collagen in nonmineralized areas.