Quantitation of the expression of cell surface antigens has hitherto been limited to analysis by either cytotoxicity tests or radioimmune assays (5, 15). We report here the use of a new methodology to analyze and quantitate the expression of mouse histocompabililty antigens (H-2 locus) in hybrid clones and parental cell types. The binding of fluorescein-tagged antibody is measured on a cell-to-cell basis in large viable cell populations using flow microfluorimetric techniques. These techniques have been used to measure hapten and immunoglobulin binding to lymphocyte populations (8, 9, 14). However, this is the first report in which these techniques have been used to examine the expression of the H-2 locus. The advantage of this approach is twofold: first, a large and statistically significant sample population may be analyzed one cell at a time, thus revealing the fine detail of heterogeneity in the expression of the cell surface markers within a population. Second, as has been demonstrated for analysis of specific components of the immune system, this method does permit fluorescence-activated sorting of cell types according to their different surface populations (8, 9, 14).

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