We have developed and tested a novel electrofusion chamber, the adjustable plate microchamber, that permits the successful electrofusion and production of hybridomas in a hanging droplet from as few as 1,000 B lymphocytes. Cell suspension volumes of 10 microliters may be used without excessive difficulty in aseptically recovering fused cells. With a modification of the hypo-osmolar electrofusion protocol with this microchamber, fusion efficiencies of the order of 10(-3) may be attained. These efficiencies are comparable to those attained with standard hardware and much higher numbers of input B lymphocytes. This technology should permit high efficiency hybridoma production using a variety of hitherto inaccessible B lymphocyte populations such as B cells isolated from human organ biopsies.