Abstract: A method for determining a connective tissue breakdown product in a body fluid from an animal includes steps of providing a standard comprising the breakdown product having a radioactive label, the standard having a known specific radioactivity, combining the standard and a sample of the body fluid, removing from the combined standard and sample a purified breakdown product fraction containing labelled breakdown product from the standard together with breakdown product from the sample, and measuring the specific radioactivity of the fraction as a measure of the quantity of the breakdown product in the sample. Also, methods for assessing a condition of a selected connective tissue in an animal in vivo, and for assessing in vivo a disease process that includes destruction of a specified connective tissue component, and for assessing in vivo the efficacy of a therapy for treatment of such a disease process, include the steps of the method for determining a connective tissue breakdown product.