A non-radioactive in situ hybridization method was used to detect residual host cells after bone marrow transplantation (BMT). A biotinylated Y-specific DNA probe was hybridized to bone marrow cells of three male patients who had received a bone marrow graft from their sisters for the treatment of acute leukaemia. Host cells were detected in two of the three patients, both in clinical remission, in six and seven cells out of 1000 cells analysed, respectively. Only one of a total of 3000 cells from three female controls showed a 'Y-signal'; 2580 cells out of a total of 3000 cells from three male controls showed a positive signal. These results indicate that in situ hybridization is a reliable and sensitive method for the detection of host cells after BMT.