Eosinophil myelopoiesis is to a great extent regulated by interleukin (IL)-5. Analysis of IL-5 mRNA in spleen cell preparations by reverse transcription-polymerase chain reaction (RT-PCR) revealed the presence of message for this cytokine in uninfected severe combined immunodeficiency (SCID) mice. This message was increased following Mycobacterium avium infection. Normal BALB/c mice had higher levels of expression of IL-5 but the expression of this cytokine was reduced during M. avium infection. Anti-IL-5 monoclonal antibody administration in vivo to SCID mice reduced the number of peritoneal and splenic eosinophils. Gamma interferon (IFN-gamma) had an inhibitory effect of eosinophilopoiesis during infection of SCID mice by M. avium since neutralization of this cytokine increased the number of eosinophils detected in the peritoneal cavity of infected animals. Our results suggest that IL-5 may be produced by cells other than T cells that are both able to respond to infection and are under the control of IFN-gamma.