When lymphoid cells from the thoracic duct lymph, blood, lymph node, spleen and thymus of the rat were exposed to pokeweed mitogen (PWM), large primitive cells appeared after 24–30 hours of culture. This response was not limited to tissues of adult animals but was also demonstrated in cultures of cells from the thymus glands, lymph nodes and spleens of newborn, 48 hour and three-day-old rats, respectively. The transformed rat cells, in contrast to the cells observed in human PWM cultures (Chessin, Bőrjeson, Welch, Douglas and Cooper, '67), could not be divided into two populations on the basis of cytoplasmic basophilia, PAS positive granules or alcian blue staining. In addition, the thoracic duct lymphocytes from rats thymectomized within 24 hours of birth showed a marked deficiency in transformation, characterized by decreased numbers of enlarged cells and minimal cytoplasmic basophilia, when compared to similar cells from non-operated or sham-thymectomized controls.

In the thymus cultures, some of the small lymphocytes served as precursors of the transformed cells without undergoing division. Subsequently, these enlarged cells proliferated giving rise to small lymphocytes and additional transformed elements. Exposing lymphoid cells to PWM for up to 14 days did not result in the development of plasma cells.