Mouse interferon preparations inhibited the antibody response to sheep erythrocytes (SRBC) in spleen cell cultures. The effect was seen when the cultures were pretreated with interferon for 6 hr, or when interferon was added up to 40 hr after addition of SRBC. The factor responsible for this inhibition could not be dissociated from the antiviral activity of interferon by standard physicochemical means. The application of a mosaic cell culture system suggested that the inhibitory effect of interferon was due to an action on B cells. Interferon treatment of T cells and macrophages did not affect their normal function as helper cells in this system. To our knowledge, these findings constitute the first example of an effect of interferon on B lymphocytes. In addition, it was found that in “low-responder” composite cultures, pretreatment of the B cell population with low amounts of interferon led to a considerable enhancement of the antibody response, whereas higher concentrations of interferon gave a similar degree of inhibition as that found for “high-responder” cultures. Late addition of mouse interferon preparations to SRBC-stimulated spleen-cell cultures augmented the number of plaque-forming cells (PFC). This effect was most pronounced when interferon was added only 4 hr before the plaques were counted on day 4. This increase was followed on day 5 by a decrease in the number of PFC compared with that of control cultures.