BALB/c mice were immunized using cells from acute monocytic leukemia patients. The BALB/c spleen cell suspension was fused with Sp2/0-Ag14 cells, and mAb 5.5 was isolated from the hybridoma cells either by radioimmunoassay with 125-I-labelled F(ab)2 fragments of sheep anti-mouse Ig as second layer, or by indirect immunofluorescence using rhodamine-conjugated goat anti-mouse Ig as the second layer.