We evaluated the harvest product from 47 patients undergoing peripheral blood (BP) stem cell collections for monoclonal plasma cells (PC) using a sensitive immunofluorescence technique. 60% (28/47) had documented tumour cells in the apheresis product. The 32 patients in plateau had a mean of 1.62 x 10(6) PC/l v 74.64 x 10(6) in 15 relapsed patients (P < 0.01). 32% (6/19) of patients without any tumour cells in the initial sample had them detected on a subsequent apheresis sample. In four cases (all treated with GM-CSF) small numbers of tumour cells were detected initially but became undetectable on a subsequent sample.