Replication of infectious bursal disease virus in lymphoid cells from the bursa of Fabricius occurs preferentially in a population of proliferating cells characterized by increased thymidine incorporation and by blast cell formation. This was demonstrated by enrichment of virus producing cells in gradients of bovine serum albumin and by mitogenic stimulation of cells with fetal calf serum or an extract prepared from bursae of susceptible chickens. Susceptibility of lymphoid bursa cells is not correlated with the expression of immunoglobulins on their surface. This conclusion was reached after the enrichment of Ig-bearing cells by rosetting techniques and by separation through a cell sorter.