Abstract: A method is known for the determination of osteocalcin in human serum or plasma, in which a sample of the biological fluid containing the osteocalcin to be determined is incubated, together with a defined amount of an oligopeptide tracer, with a suitable anitbody which binds both the osteocalcin and the tracer. This method has proved to be susceptible to error in that the incubation conditions affected the measured levels, and the osteocalcin levels obtained were often too high. The recognition that the observed errors are attributable to a proteolytic breakdown of the oligopeptide tracer by constituents of the serum or plasma resulted in the technical teaching of the addition of one or more proteolysis inhibitors to the sample of human serum or plasma before or together with the addition of the oligopeptide tracer, specifically either a mixture of an endopeptidase inhibitor with an aminopeptidase inhibitor or, when an N-terminally protected oligopeptide tracer is used, only an endopeptidase inhibitor.