2 patients with acute leukaemia in relapse were transplanted with fetal liver cells following a conditioning regimen of cyclophosphamide (120 mg/kg) and total body irradiation (1000 r). Each patient achieved a remission with haematopoietic recovery that was rapid in one case and delayed in the other. In one case there was evidence of chimerism as demonstrated by the presence of the XYY karyotype of the donor fetus in 20% of marrow metaphases, by the presence of double Y bodies in the peripheral blood, by the appearance of new HLA‐antigens, and by red cell isoenzyme phenotypes of donor origin. In the second case there was prompt haemotopoietic recovery and the appearance of red cell isoenzyme phenotypes of donor origin. Survival was 153 and 30 d, respectively, and both patients died of interstitial pneumonia without evidence of graft versus host diseas