PHA-M stimulated lymphoblasts obtained from peripheral blood and separated from small lymphocytes by X 1 g velocity sedimentation, unstimulated blood lymphocytes, monocytes and cells isolated from the bursa of Fabricius of chickens, were infected in vitro by the pathogenic strain CU-1 of infectious bursal disease virus (IBDV). Six hours after infection 32.5 per cent of the bursal cells reacted immunocytologically with IBDV antiserum and had high infectivity titers in plaque assays. Separated lymphoblasts showed a marked lower degree of virus replication and only 2.5 per cent reacted positively when studied by immunocytology, while monocytes ranged between these two cell types with regard to both the degree of virus replication and the positive reaction with IBDV antiserum. Small lymphocytes, however, were found to be totally resistant to IBDV infection. When studied by electron microscopy, virus particles arranged in a crystalloid pattern could only be detected in bursal cells. The results of this study indicate that proliferating lymphoid cells at a certain stage of cellular differentiation are the target cells for IBDV, and that in infected chickens monocytes may play a role in the spreading of the virus.