One of the major problems using man-mouse chimeras is still the difficulty to demonstrate unequivocally engraftment of human cells in murine tissues. Using supravital labelling of human leukocytes with the Hoechst dye H33342, it was possible to demonstrate directly their engraftment and to assess their distribution in the tissues of the severe combined immunodeficiency (SCID) mice. The human cells can be traced for a period of 4-5 weeks. In contrast to earlier reports, combined marker and labelled-cell studies suggest that T-cell surface marker CD3 with reported specificity for human lymphocytes are indeed found, also in man-mouse chimeras, only on human cells. The ratio of B and T cells of human origin changes significantly after transfer into SCID mice and differs among various SCID tissues. The simple staining procedure using the supravital nuclear dye H33342 opens new possibilities for the study of cellular interactions and host responses of the human immunoreactive cells in an increasingly well-characterized animal model.