Synthesis of DNA in lymphocytes isolated from adult human peripheral blood was detected by measuring incorporation of tritiated thymidine after cell cultures were exposed to polyhedra of a nuclear polyhedrosis virus ofAutographa californica. Similar results obtained with lymphocytes isolated from human umbilical cord blood suggests that this mitogenic activity is nonspecific. No DNA synthesis was evident when lymphocyte cultures were treated with enveloped or unenveloped nucleocapsids. Observation of lymphocytes cultured over a 3D-day period, in the presence of baculovirus, provided no evidence of permanently transformed cells.