Severe combined immunodeficient (scid) mice of the inbred strain C.B-17 lack functional T and B cells and, because of this, they tolerate xenografts. We reconstituted scid mice with human peripheral blood leukocytes (PBL) by i.p. injection. In order to determine the human PBL in lymphoid organs of these reconstituted scid mice, we labelled the human PBL prior to transfer with the fluorescent dye PKH 26-GL. With this experimental approach it was possible to detect the human cells in lymphoid organs and peritoneal exudate of the reconstituted scid mice by cytofluorimetrical and histological methods. This method is thus helpful for the determination of xenografts transplanted upon scid mice as well as in other experimental settings including adoptive transfer.