Detection of interleukin-3 in the serum of mice infected with Mycobacterium lepraemurium.
 Infection of mice by Mycobacterium lepraemurium is accompanied by ablation of erythropoiesis in the bone marrow and gross enlargement of the spleen.
 This, together with increased monocytopoiesis and the earlier demonstration of macrophage colony-stimulating factor in the serum of infected mice, suggested the activity of additional cytokines.
 Eight weeks after infection of mice by M.
 lepraemurium, interleukin-3 (IL-3) activity was demonstrated in the serum (titer, 1:3200).
 The serum titer of IL-3 activity was maximal after 13 weeks (greater than 1:6400) and was slightly reduced after 18 weeks (1:6400).
 That the IL-3 activity detected in the serum of the M.
 lepraemurium-infected mice reflected the presence of IL-3 itself was confirmed by a neutralization assay using anti-murine IL-3 antibodies; IL-3 activity in the serum of mice 13 weeks after infection was completely abolished by the anti-IL-3 antibodies.
 Finally, a 1-kb signal of IL-3 RNA was detected in the spleens of M.
 lepraemurium-infected mice 13 weeks after infection.
