Among the substances causing an unspecific stimulation of lymphocytes in vitro pokeweed mitogen (PWM), a glycoprotein isolated from pokeweed (Phytolacca americana L.), is of special interest. Studies performed at the light and electron microscope level have shown the production in vitro by PWM of large phytohemagglutinin (PHA) -like blasts and of a unique intermediate sized cell type exhibiting a well developed rough-surfaced endoplasmic reticulum and ribosomal aggregates. These plasmacytic cells have also been observed in the peripheral blood of humans who sustained systemic exposure to pokeberries. Further studies regarding the functional properties of these cells are required to determine whether or not PWM is able totransform lymphocytes into true plasma cells in vitro and in vivo.