Calibration curves are determined for measuring the concentration of 10B in the blood of rats using an autoradiographic procedure, with the polycarbonate solid-state nuclear track detector CR-39 and an image analysis system for automatic track counting. The calibration curves indicate that for the etch procedure used, the nitrogen concentration in the blood is an important interfering input for 10B concentration measurements. By discriminating against small tracks, the sensitivity to the blood nitrogen concentration can be reduced to the point that a variation in the nitrogen weight fraction of 0.01 gN/g blood causes a variation in the predicted 10B concentration of 0.3 µg 10B/ml blood.