Mixed chimerism has been demonstrated in organ transplant recipients surviving over a long period. However, little is known about the fate and movements of donor cells following liver transplantation. In this study, using rat male-to-female liver transplantation, we assessed microchimerism by semiquantitative polymerase chain reaction (PCR). A PCR specific for the Y-chromosome (sex-determining region Y [Sry]) allowed the distinction of small amounts of male cells in a large excess of female cells. Nonimmunosuppressed Lewis recipients of Lewis liver grafts survived > 60 days, and donor cells were detected in peripheral blood up to day 60 in four of six recipients. All immunosuppressed Lewis recipients survived for 60 days, and the donor cells were detected up to day 60. Semiquantitative PCR demonstrated that in immunosuppressed Lewis recipients surviving > 60 days with no acute rejection episode, the donor-to-recipient cell ratio remained > 1:1,000. In nonimmunosuppressed Lewis recipients of ACI liver allografts, donor cells in peripheral blood decreased rapidly by day 7. In immunosuppressed Lewis recipients of ACI liver allografts, donor cells were detected for longer periods, in proportion to the amount and duration of FK506 (Fujisawa Pharmaceutical Co., Osaka, Japan) administered. However, in recipients with deteriorating function due to ongoing rejection, small amounts (0.01 percent) of donor cells were detected. Our results indicate that the Sry-specific PCR is useful for assessing microchimerism following rat liver transplantation, and demonstrate correlation between microchimerism and graft outcome; high- level microchimerism (>0.l percent) correlated well with graft acceptance, whereas low-level microchimerism (<0.01 percent) did not.