We used U937 cells 3 days after passaging. Half were activated with IFN-gamma 100ng/ml for 20 hours and half were used as control. Cell concentration at the beginning of activation was 2.105 cells/ml. Brefaldin A was used as an intracellular blocking agent at 1ug/ml, in contact with the cells for 4 to 6 hours. The graph on the left represents activated U937 cells marked with B-C50 and the graph on the right shows B-C50 with non-activated U937 cells. In the two cases, the dotted line represents the IgG2a isotype control (B-Z2).