The pituitary hormone thyrotropin (TSH) has been shown to enhance in a dose dependent manner the in vitro antibody response. Highly purified preparations of bovine and human TSH enhanced up to 375% the number of cells producing antibody to sheep erythrocytes. TSH had to be present prior to 24-48h of the initiation of culture for enhancement of the antibody response. An analogy is discussed between TSH and B lymphocyte growth and differentiation factors. © 1984 Academic Press, Inc. All rights reserved.