Measles virus infection is associated with suppression of immune functions both in vivo and in vitro. The virus infects T lymphocytes, B lymphocytes, and monocytes, but does not produce cytolysis. One consequence of infection in vitro is the failure of T and B lymphocyte mixtures to cooperate in secreting Ig in a PWM-driven system. Here we report that this defect in Ig secretion resides in the infected B lymphocyte, but not in the T lymphocyte or monocyte. Further, NK cells are not involved, since neither their depletion nor reconstitution abrogates suppression of B cell function. Proliferation of B cells in the early culture period is suppressed, suggesting that measles virus suppresses B cell development at the activation or proliferation stages, but does not affect terminal differentiation into Ig secreting cells.