Transglutaminase was detected either at the single cell level by fluorescent staining with dansylcadaverine or in cell homogenates by incorporation of [14C] putrescine into alpha-casein. In the mouse it was found that erythrocytes, granulocytes, thymocytes or lymphocytes with or without concanavalin A stimulation were negative in the fluorescence test. Normal peritoneal washout macrophages and peritoneal exudate cells stained positive to varying degrees (induced with mineral oil 64%, with thioglycollate 50%, with proteose peptone 22%, normal washout 1%). Macrophages from bone marrow liquid cultures were 20% positive at day 3 and 100% at day 17. Promonocytes and monocytes were negative. Positively stained cells also phagocytosed opsonized sheep erythrocytes. The degree of staining varied considerably in the macrophage-like cell lines IC21 (100%), J774.2 (75%), P388-D1 (50%). This result and those from autoradiography studies indicate that expression of transglutaminase is not associated with the S-phase of the cell cycle. The fluorescence test correlates quantitatively with the [14C] putrescine incorporation test. The enzyme is Ca2+-dependent and appears neither to be on the outer cell surface nor being released into the culture medium. Circumstantial evidence indicates that it is also not compartmentalized in cytoplasmic vesicles. While the induction and modulation of enzyme expression is still under study, it is concluded that transglutaminase is a new marker for macrophages of a certain differentiation or activation state.