Detection of micrometastatic cells in bone marrow (BM) may potentially be of prognostic value in colorectal cancer (CRC). In the present study, we have evaluated our immunomagnetic detection method in model experiments and on BM samples from CRC patients. In repeated experiments, 11 of 12 CRC cell lines consistently bound MOC31 antibody-coated magnetic particles with an average of 98% of the cells being rosetted with the beads. When different numbers of CRC cells (20, 100, 200, and 1000) were admixed to 1 x 10(7) mononuclear cells (MNCs) from BM, a mean of 77% of the cancer cells was recovered. In BM samples obtained from CRC patients at primary surgery, rosetted tumor cells were detected in 46 of 275 samples (17%) upon screening of 2 x 10(7) MNCs/sample. The fractions positive were: 10% (5 of 49) in Dukes' A; 17% (20 of 115) in Dukes' B; 23% (18 of 78) in Dukes' C; and 9% (3 of 33) in Dukes' D. Of 206 control samples, three (1.5%) contained cells in BM that formed rosettes with the MOC31 beads. In positive samples, a median of eight tumor cells (range, 2-120) were identified per 20-microl examined fraction, representing about one-tenth of the total sample. The results demonstrate the feasibility of using the immunomagnetic method for detection of micrometastatic CRC cells. Furthermore, that screening of 2 x 10(7) MNCs in a BM sample can be completed in <3 h makes the method an attractive alternative to other techniques.