At is important to know if a single im munocyte can synthesize simultaneously two antibodies to two distinctly different antigens, because such knowledge would carry implications affecting theories on antibody formation. To obtain informa tion on this question, several techniques have been developed for studying antibody formation by a single cell. Attardi (1) and Nossal (2, 3, 4, 5, 6) used the sensitive microdrop method and demonstrated that single cells contained antibodies to different antigenic determi nants on separate particulate antigens. While this appeared not to be a wide spread phenomenon, as most of the cells contained antibodies to only one or the other antigen, this finding must be incor porated into a suitable theory of antibody formation. Coons (7), White (8), and Vazquez (9) obtained different results, using primarily soluble antigens. They showed that when animals were inoculated with two different