Bone marrow of mice parasitized with Nippostrongylus brasiliensis showed increased numbers of eosinophils as early as 4 days after infection. By day 7, their bone marrow also contained elevated numbers of progenitors that form small eosinophil colonies (20 to 50 cells) in soft agar cultures supplemented with interleukin-5 (IL-5). However, when mice were infused with anti-IL-5 antibodies at the time of infection, the number of recognizable eosinophils present in bone marrow remained low and eventually dropped below normal levels. The antibody treatment also prevented increased generation of IL-5- responsive precursors capable of differentiating into mature eosinophils in liquid culture and inhibited the generation of progenitor cells capable of forming small eosinophil colonies or clusters in soft agar cultures. The results of these in vivo experiments directly show that IL-5 is an essential regulatory molecule required for the bone marrow-dependent phase of a parasite-induced eosinophilia.