An avidin-biotin alkaline-phosphatase (ABAP) staining method has been developed for the labeling of tissue sections and cell smears. The introduction of alkaline phosphatase as a marker enzyme through an avidin bridge results in excellent immunocytochemical labeling of different antigens using poly- and monoclonal antibodies. This technique avoids problems with endogenous peroxidase activity that sometimes occur using peroxidase staining procedures. The introduction of a preformed avidin-biotin alkaline-phosphatase complex (ABAPC) makes the presented technique as simple to handle as the widely used avidin biotin-peroxidase complex method (ABC). The ABAPC technique could be combined with other enzymatic labelings for double immunoenzymatic staining.