A sensitive modification of a reversed hemolytic plaque assay demonstrating individual immunoglobulin-secreting cells (ISC) is presented. The test is performed by adding test cells to a monolayer of indicator sheep erythrocytes coated with an IgG fraction of rabbit antiserum against the F(ab')2 portion of human immunoglobulins (Ig). After having added a developing antiserum and complement, hemolytic plaques with a central ISC could be counted in an inverted microscope. In addition, when ISC were killed and fixed in situ in the monolayer, the cells could be further studied by immunofluorescence of their cytoplasmic Ig.