Turkey poults which were surgically or chemically bursectomised after hatching, and inoculated with the lymphoproliferative disease (LPD) virus at 31/2 weeks of age, developed typical tumourous lesions in various organs (pancreas, spleen, thymus, liver, gonads and kidneys) to the same extent as intact but inoculated controls. Plasma virus-associated reverse transcriptase activity (as an estimation of viraemia) developed at a higher rate in poults neonatally treated with 16 mg of cyclophosphamide. The chemically bursectomised birds were found to have markedly reduced serum gamma-globulins levels, and low levels or absence of agglutinins to sheep red blood cells and to killed Brucella abortus following immunisation with these antigens. Inoculation of turkey poults with LPD virus did not cause inhibition of the humoral immune response in intact birds but reduced significantly antibody production in surgically bursectomised poults. Since infection with LPD virus was previously shown to cause hypergammaglobulinaemia, and more specifically, a marked increase in serum IgG (7S) levels, it was suggested that the LPD tumour cells might be antibody-producing B-lymphoid cells. However, results presented here indicate that LPD lesions and viraemia can develop even in turkeys lacking any appreciable B-cell activity.