The authors' method depends on labelling in vivo the zone of demarcation of actively forming osteons by tetracycline antibiotics. The drugs, thus deposited in bone, are visualized by fluorescence microscopy. The label is a circular or elliptical ring whose diameter indicates the degree of completeness of the osteon at the time of drug administration. A histogram of the ring diameters therefore classifies labelled osteons as regards stage of completion at the time of drug administration. If an average formation time is assigned to osteons measured, conclusions relative to the rate of formation of the average osteon can be derived.In a histogram plotted from more than 900 labelled osteons in bones of three patients, most of the labels were concentrated in the inner half of the osteon. The authors conclude that osteon formation is initially rapid both in terms of cubic microns of bone formed in a unit time and in terms of decrease in diameter of the Haversian canal; it decreases with passage of time, and it ends with a critical minimum diameter of the Haversian canal. A 2-week cyclic variation in rate may be superimposed on the overall plot.