Lymphocytes from bursa of Fabricius and thymus of chickens were purified and separated into the three cell subsets--T, B, and null cells--by the techniques of nylon fiber columns and cytotoxicity tests. The in vitro susceptibility of the fractionated lymphocytes to a virulent strain of infectious bursal disease virus (IBDV) was studied by using immunofluorescence as the infection criterion. B cells were highly susceptible. By contrast, T cells and null cells were insusceptible to infection by IBDV. The relationship between the target cells for virus infection and those B cells that possessed surface immunoglobulin (SIg) was tested. B cells were further divided into SIg(M)- and SIg(G)-bearing cells by immunoadsorbent columns employing anti-immunoglobulin M(IgM) (mu-specific) or anti-IgG (gamma-specific) sera coated with Sephadex. The SIg(M)-bearing cells were highly susceptible. These results suggest strongly that SIg(M)-bearing B cells were the target cells for infection by IBDV.

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