The results from this investigation suggest that chemotactic factor(s) from damaged myofibers attract polymorphonuclear leukocytes (PML) and macrophages to the site of injury, while exudate macrophages but not PML induce a strong positive chemotactic response in myogenic cells. The AB and BB isoforms of platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-beta), basic fibroblast growth factor (bFGF), and leukemia inhibitory factor (LIF) (all of these are secreted by macrophages) were also shown to be chemoattractants for muscle precursor cells (MPC). The AA isoform of PDGF did not appear to have any such chemotactic effect on MPC. Macrophages were also shown, under tissue culture conditions, to stimulate the proliferation of MPC. This mitogenic effect was similarly demonstrated for the BB isoform of PDGF, bFGF, and TGF-beta but not for the AA or AB isoforms of PDGF nor for LIF. The results indicate that macrophages play a pivotal role in the orchestration of muscle fiber reconstitution.