Even though colorectal cancer tends to aggregate in families, there is paucity of information on the genetic determinism for familial colorectal cancer (FCRC) predisposition. Therefore, we investigated constitutional chromosome abnormalities and bleomycin induced chromosome sensitivity of 26 familial and 30 sporadic colorectal cancer (SCRC) patients, 60 unaffected family members (first/second degree relatives) and 30 normal healthy controls to determine whether these parameters could give any clues on genetic predisposing factors by which high risk members in CRC families could be identified. The test assay used bleomycin-induced chromatid breaks in short term microcultures of peripheral blood lymphocytes of the subjects. The CRC patients, the unaffected family members and the controls did not show any constitutional chromosomal abnormalities. However, with regard to bleomycin sensitivity, there was significant difference between the CRC patients, unaffected relatives and controls. The mean b/c values of 1.64+/-0.42 for the FCRC patients and 1.08+/-0.34 for the SCRC patients were significantly higher than the mean b/c values of 0.62+/-0.18 for the unaffected relatives and 0.52+/-0.12 for the controls (P<0.001). A noteworthy observation was that 6 unaffected members from 6 CRC families also showed bleomycin hypersensitivity, at the initiation of the study. Since they expressed mean b/c values greater than 1.0, which was as high a value as those of the patients, they were regularly followed up. Out of these 6 members, 2 developed CRC later. This clearly demonstrates that mutagen hypersensitivity among unaffected relatives in CRC families may be related to cancer predisposition. Hence, this cytogenetic assay could be utilised to identify the genetically high risk individuals in CRC families.