Fifty-eight samples of bone marrow (31), whole peripheral blood (8) and separated fractions of circulating mononuclear (11) and polymorphonuclear (8) cells from 18 male patients, transplanted for hematological diseases from related (14) or unrelated (4) female donors were analyzed for chimerism at subsequent intervals (range, 1-72 months) following bone marrow transplantation, by means of PCR amplification of the Y-chromosome-specific DYS14 sequence, revealed by a radiolabelled hybridization probe (dot blot technique, 0.01% sensitivity). Detection of male cells was positive in all but two of 52 samples collected within the third year after transplantation and negative in six samples collected from three patients after the third year. In the first year after transplantation, mixed chimerism was found in all patients, apparently with no correlation with graft-versus-host disease. Comparable results were found in fractions of mononuclear and polymorphonuclear cells, when analyzed separately. The persistence of very low levels of recipient cells in patients in continuous complete remission until the third year after transplantation, suggests the persistence of normal host hemopoiesis for a long period of time after the so-called myeloablative regimen. The progressive negativization, occurring in our patients between the second and the fourth year after transplantation, could signify the disappearance of residual host hemopoiesis or its decrease to below the detection level of this highly sensitive method.