In assays of leukocyte function it is desirable to correlate a specific measurement, e.g., the release of enzymes, the production of metabolites or the secretion of cytokines with the amount of cellular material. Many authors use either the protein or the DNA content of the cells as a reference. Specific measurements of cellular activity are often expressed as a ratio of activity/protein or activity/DNA. This communication describes a dye-binding assay which allows the determination of total protein of the same cell sample which was first used for the quantitation of a specific cellular function. The microplate assay described is an adaptation of the Coomassie blue dye-binding assay without the need of a separate step to lyse the cells. The test is useful for both adherent and non-adherent cells and has a detection limit of less than 500 ng BSA or 5000 leukocytes.