The surface expression of CD117 on plasma cells (PCs) from normal individuals and patients with multiple myeloma (MM) has been analysed using triple-stained cells for flow cytometry. In addition, the clinical significance of CD117 expression in MM patients and its possible value for the evaluation of minimal residual disease was explored. A total of 11 healthy volunteers and 56 untreated MM patients were included in the study. The expression of CD117 was analysed by flow cytometry, using simultaneous staining with the MAbs BB4, CD117 and CD38. Cell acquisition was performed in two consecutive steps using a live gate drawn on SSC/CD38 cells and a total of 300,000 events were acquired. For data analysis, the Paint-a-Gate Plus software (Becton Dickinson) was used. PCs were identified according to their strong reactivity for CD38 and their positivity for BB4, as well as by their light scatter distribution. Dilution experiments of CD117+ myelomatous PCs with normal bone marrow (BM) cells were performed in order to assess the sensitivity level of the technique for detection of CD117+ residual PCs. None of the PCs from normal BM samples showed reactivity for the CD117 antigen. In contrast, CD117 antigen was present in 18/56 MM patients (32%), the proportion of positive cells in these cases being as high as 92.1 +/- 9%. Therefore, within PC lineage the c-Kit antigen would be restricted to the myelomatous population and thus could be considered as a 'tumour-associated marker' for monitoring minimal residual disease in about one third of MM patients. Dilution experiments indicate that the detection limit with this marker would be 10(-4) (one myelomatous PC/10(4) normal BM cells). Upon comparing the clinical and haematological disease characteristics of CD117-positive and CD117-negative cases, no significant differences were found.