In bone matrix, type I collagen is stabilised by covalent cross-links formed between adjacent collagen molecules; the majority of which is believed to be immature, divalent bonds. For studying these immature forms in detail, we have developed an immunoassay for a synthetic peptide SP 4 that is analogous with and detects a linear epitope within the C-telopeptide of alpha1-chain of type I collagen. The SP 4 assay, together with the ICTP assay, which is specific for the trivalently cross-linked C-telopeptide, was used for the isolation of the differently cross-linked C-telopeptide structures of the alpha1-chain of type I collagen present in mineralised human bone. Amino acid analysis, peptide sequencing and MALDI-TOF mass spectrometry were used to identify and characterise each of the isolated structures. The cross-link content of each isolated peptide was identified. In the trivalent ICTP peptide, only 40% was cross-linked with pyridinoline, the remainder of the cross-links being currently uncharacterized. The divalent peptides contained only previously characterised cross-linking structures. Most of the divalent cross-links were dihydroxylysinonorleucine (DHLNL), with minor amounts of hydroxylysinonorleucine (HLNL). The relative proportion of the HLNL cross-link was slightly higher in the divalent alpha1Calpha2H peptide. A substantial amount of uncross-linked telopeptide structures was also found. Previous studies, where direct chemical cross-link analyses have been used to assess the maturity of cross-linking, have inferred that bone contains more divalently than trivalently cross-linked C-telopeptides. The immunochemical peptide approach used in this study may help to detect presently uncharacterized, trivalent cross-links, the presence of which is strongly suggested in this study. It also provides additional information regarding the extent and maturity of tissue type I collagen cross-linking in health and disease.