Direct measurement of monoclonal plasma cell mass in bone marrow biopsies may be a useful parameter to establish in plasma cell dyscrasia. In this study monoclonal plasma cells/mm in light chain immunoglobulin immunostained archival bone marrow sections from 22 patients in whom a diagnosis of multiple myeloma (MM) had been excluded but who had monoclonal proteins were counted by two observers at light microscopic level. There was good correlation between the counts of the two observers. The levels of monoclonal plasma cells/mm in biopsies were not related to the % counts in the aspirates taken at the same time as the biopsies. Three of seven patients with biopsy levels in excess of the polyclonal levels in patients without plasma cell dyscrasia developed progressive MM within the observation time. Monoclonal plasma cell levels/mm of bone marrow biopsies can be measured and they provide a useful parameter for the assessment of patients with low volume plasma cell dyscrasia.