We report that basophils in peripheral blood can be stained using histamine immunocytochemistry. The staining is based on the fixation of leucocytes with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (CDI) and the subsequent incubation of these cells with antisera raised against histamine conjugated to different carrier proteins using CDI. The staining appears to be specific for basophils and stained cells can be examined using both fluorescence microscopy and flow cytometry. In addition, histamine immunocytochemistry can be combined with conventional immunocytochemistry by incubating leucocytes with antibodies to cell surface antigens prior to or following fixation of the cells with CDI. Thus, histamine immunocytochemistry may be a valuable tool in future studies of human basophils.