Transplacentally acquired lymphoid chimerism was detected in two infants with severe combined immunodeficiency (SCID) by two-colour cytofluorographic studies. These cells had no demonstrable function in studies in vitro. Following T cell-depleted maternal bone marrow stem cell transplantation, evidence of T cell function was detected 20 and 50 days later, and transient B cell function was detected 50 days later. These immune functions appeared much sooner than the 90-120 days usually required for T cell function and the 2-2.5 years for B cell function to develop after haplo-identical stem cell transplants into SCID infants without transplacental engraftment. The presence of maternal lymphoid chimerism did not interfere with haplo-identical marrow cell engraftment, even though no pre-transplant immunosuppression was given. This observation suggests that the transplanted maternal marrow stem cell in some way conferred reactivity on the engrafted but apparently non-functional mature T cells that had entered the fetal circulation transplacentally.