Prothymosin alpha (ProT alpha) distribution in human and rat lymph nodes and human tonsils was studied by means of immunohistochemical methods, using specific antibodies raised against thymosin alpha 1. We observed ProT alpha immunoreactivity in lymphoid cells of the germinal centers both in humans and rats. In human tonsils, positive cells were also seen in the basal layer of the surface epithelium. These results support the hypothesis that ProT alpha expression is restricted to actively proliferating cells. Immunoelectron microscopy revealed that ProT alpha was located in the nucleus, mainly in the border between euchromatin and heterochromatin.