We describe a new liquid cell culture system for propagating Burkitt lymphoma (BL) cells contaminating the bone marrow (BM). The percentage of BL cells at day 10 of culture is correlated with the number of BL cells in the initial sample, which permits the detection of as few as one BL cell in 10(6) normal cells. We currently use such an assay to detect minimal BM involvement in our patient's follow up. This has allowed to define in which clinical situations the BM collected for autologous transplantation (though cytologically normal) had to be "purged". With the help of this modified liquid system it was possible to quantify the efficacy of various purging procedures on different models; an experimental model of allogenic irradiated BM contaminated with BL cell lines was used. Pre-clinical assays were also conducted on our patients' BM contaminated with their own tumour cells, or on invaded BM collected for autologous transplantation and for which the system had evidenced the efficacy of the purging procedure.