Supernatant fluids from cultures of sensitized lymphocytes incubated with specific antigen contain a soluble factor which lowers the contact angle between a sessile drop of buffered saline and macrophage monolayers. This effect, which reflects a reduction in the interfacial energy of the cell surface, is carrier specific and is not due to antibody or antigen in the culture fluids. This factor appears concomitantly with MIF production. The lowering of contact angle is associated with enhanced phagocytosis, in a system involving opsonized erythrocytes. Aside from providing information about interactions between sensitized lymphocytes and macrophages, this system may serve as a rapid assay for detecting lymphokine activity.