Peripheral blood samples from healthy adults, patients with transplanted bone marrow, and healthy children were analyzed for the proportions of cells positive for CD2, CD4, CD8, CD19, CD56, CDw65, and KiM8 cell membrane markers. Six cell gates were defined on granularity vs. cell size (L90 degrees LSx-FALS) flow cytometric display for each blood sample: large lymphocytic, small lymphocytic, monocytic, mononuclear, granulocytic, and all-cell gate, and the listed markers were determined in each of them. Considerable differences in marker-positive cell proportions were found between healthy adults and children, and particularly between healthy adults and patients with transplanted bone marrow. Marker-positive cell frequencies corresponded to biologic distribution of three leukocyte populations within the defined gates, in a general agreement with known specificities of the antibodies used. A considerable degree of variations in the positions of gates drawn for different samples and the numbers of cells counted in them was observed. Still it appeared that all six gates, otherwise commonly used in flow cytometric analysis, could be precisely defined and yielded reproducible data. large and small lymphocytic gates yielded very similar marker frequencies, revealing that in cases where three leukocyte populations were not clearly delineated on (90 degrees LSxFALS) display, a smaller lymphocytic gate could be safely drawn in order to avoid contamination with monocytes.