Tumour cell mass (TCM) in patients with multiple myeloma has been measured by synthetic rate studies of bone-marrow tumor cells in culture. From the measurements of Salmon and co-workers, a simple clinical staging system in which TCM is calculated by a programmable pocket calculator has been developed. We have compared the measurement and calculation of TCM in patients with IgG and IgA multiple myeloma by these methods in our own laboratory. There was correlation between synthetic rate and serum IgG paraprotein concentration but not the serum IgA paraprotein concentration. In patients with IgG myeloma there was correlation between measured and calculated TCM as well as measured TCM and serum IgG paraprotein concentration, urine Bence-Jones protein excretion, and serum albumin concentration. Measured TCM also inversely correlated with haemoglobin concentration. In patients with IgA myeloma, however, there was no correlation between measured TCM and calculated TCM or any other individual clinical laboratory parameter. A number of problems have contributed to the overall failure of this method to measure and calculate TCM including methodological differences, definition of myeloma cells, proteolysis of newly synthesized paraprotein in culture, changes in synthetic rates with time and assessment of the degree of bone lesions by skeletal roentgenograms. We cannot recommend these methods as they stand to measure and calculate TCM in patients with multiple myeloma.