Use of a probe to repeat sequence of the Y chromosome for detection of host cells in peripheral blood of bone marrow transplant recipients. In situ hybridization for the Y chromosome (Y-ISH) was used to identify residual host cells in the peripheral blood of 51 recipients of sex-mismatched allogeneic marrow not depleted of T cells following conditioning with high-dose cyclophosphamide and total body irradiation (TBI). One patient who rejected the graft showed rapid reappearance of host cells after transient donor marrow engraftment. Host cells were detected at low levels in 49 of the remaining 50 patients. Host peripheral blood mononuclear cells (PBMC) decreased with time and reached a plateau at 1.0 +/- 0.2% within four weeks after transplantation, while the percentage of host granulocytes (GRAN) reached a plateau at background level. The mean absolute numbers of host PBMC or GRAN were less than 0.015 x 10(9)/L and did not vary significantly over the period studied. Neither the percentages nor the absolute numbers of host PBMC or GRAN were significantly affected by HLA-matching, TBI dose-intensity, pretransplant remission status, subsequent development of acute or chronic graft-versus-host disease or relapse after transplantation. The authors conclude that it is common to find a few residual host cells in the peripheral blood of allogeneic marrow transplant recipients, and the presence of these cells has no clinical significance. 