1. One-hundred and ten double-tracer (47Ca/45Ca or 47Ca/85Sr) calcium-absorption studies were performed in patients with a variety of metabolic bone disorders. A computer program was written to fit plasma radioactivity data from both oral and intravenous tracers by spline functions and to deconvolve points interpolated from the fitted curves to obtain the spectrum of *Ca transit times from mouth to plasma.
2. Methodological precision in the estimation of fractional absorption, maximum and mean rates of absorption and mean transit time was not substantially degraded by the normal uncertainties of sample counting.
3. From the results of 35 studies in which 85Sr was given as an additional intravenous tracer, it is concluded that the use of oral 47Ca and intravenous 85Sr is an acceptable alternative to the 47Ca/45Ca combination and offers considerable practical advantages.
4. The single-tracer whole-body retention and faecal collection methods for estimating calcium absorption were found to be sensitive to variations in endogenous excretion and bone accretion of calcium, making these methods unsuitable for some purposes.
5. The single-tracer method of Marshall and Nordin (1969) provided reasonable estimates of absorption rates, but the simplifications inherent in this model give rise to unpredictable systematic uncertainties in the calculation of mean absorption rates.
6. The double-isotope (47Ca oral, 85Sr intravenous) calcium-absorption test, in which blood is sampled serially over 6 h, is the method of choice for the detailed investigation in vivo of the physiological mechanisms of radiocalcium absorption by the undisturbed gut.