Inflammatory lung diseases are associated with destruction of interstitial collagen and release of degraded collagen fragments into the lower respiratory tract. To determine whether degraded collagen might be one factor mediating the cellular influx, we measured polymorphonuclear leukocytes (PMN) in hronchoalveolar lavage fluid following intratracheal instillation of collagen peptides. The responses to collagenous peptides derived from collagen digested with bacterial collagenase and to the collagenous-like polytripeptide (proline-proline-glycine)\ were examined. Both types of collagen peptides were chemotactic for human PMN in vitro. Two days following instillation of 5 mg collagenous peptides, we observed a threefold increase in the percentage of PMN in lavage fluid with a maximum (fivefold) response on day 6. Since other chemoattractants produce a response within hours when instilled intratracheally, we postulated that the late neutrophil influx produced with collagen may be the result of production of a neutrophil chemoattractant by alveolar macrophages. Alveolar macrophages treated with collagenous or collagenous-like peptides released PMN chemotactic factors, and the time course of release of chemotactic activity by alveolar macrophages in vitro correlated with the in vivo finding of a 2-6-day delay in PMN accumulation in the lungs. These observations are consistent with the idea that collagen peptides may stimulate alveolar macrophages to produce chemotactic factors for neutrophils. This mechanism may play a role in the accumulation of phagocytic cells in the lung following injury.