A new microtechnique for detection and titration of antibodies cytophilic for homologous macrophages is introduced and experiments designed to determine the optimum conditions for its performance reported. The technique employs a series of small chambers formed on a microscope slide by the application of a plastic film in which holes have been punched. The procedure uses 5 x 10(4) cells and 25 microliter of serum per chamber, and each test can be completed in 2 h. A permanent preparation results which can be examined as and when convenient.