Abstract: A method of assaying bone collagen degradation activity in a human subject. In the method, a human urine sample is reacted with an antibody which (i) is capable of reacting immunospecifically with pyridinium crosslinks selected from the group consisting of native free pyridinoline and native free deoxypyridinoline, and (ii) has a ratio of reactivity toward the selected pyridinium crosslinks and urinary pyridinium peptides larger than 1,000 daltons in molecular weight, of greater than about 5:1. An immunocomplex forms between the antibody reagent and the selected pyridinium crosslinks which are present in the sample, and the amount of immunocomplex is measured. Also disclosed are antibody reagents and kits which can be used in the method.