Enhanced chromosomal radiosensitivity in breast cancer patients has been demonstrated in several studies. To investigate the chromosomal radiosensitivity of lymphocytes in breast cancer patients the G2 and micronucleus (MN) assays are often used. In these assays blood samples are exposed to ionizing radiation and the number of radiation-induced micronuclei or chromatid breaks are scored. In most studies investigating the in vitro chromosomal radiosensitivity of breast cancer patients the G2 and MN assays were performed on freshly drawn blood. The disadvantage of working with fresh blood samples is that in most cases only one blood sample can be obtained and that the assay cannot be easily repeated without further blood sampling. To allow repeated testing we propose the use of long-term cultures of T lymphocytes (IL-2 cultures). In this study we therefore investigated whether the radiation-induced MN response in IL-2 cultures was the same as in concordant whole blood cultures. For this study the MN assay (2 Gy) was performed on IL-2 cultures of 11 sensitive breast cancer patients and 20 healthy women. The results demonstrate that the enhanced chromosomal radiosensitivity observed in whole blood cultures of breast cancer patients is not present in IL-2 cultures derived from the same blood samples. Therefore, care has to be taken when IL-2 cultures are used to assess chromosomal radiosensitivity in breast cancer patients.