The role of polymorphonuclear leukocytes (PMNs) in fibrotic lung disease remains poorly defined, but PMN prominence may predict a poor response to therapy in some patients. The bleomycin injury to rats represents a model of PMN-prominent alveolitis leading to fibrosis. We examined a method of identifying the alveolitis during its PMN-prominent phase with indium 111-labeled homologous PMNs (In-PMNs). When pairs of injured and uninjured rats were compared, greater activity was consistently found persisting in the lung images of the injured rats, both several hours (early) and a day (late) after injection of In-PMNs. Differences could likewise be identified when lung activity was normalized to activity of the injectate, the spleen, and variably the liver, as determined from organ images. These differences in gamma camera images were supported by direct measurement of activity in organs obtained by killing the animals after scanning. When groups of animals were compared, lung/spleen ratios appeared to most consistently differentiate the group of normal animals from those with alveolitis. These methods may provide a noninvasive means of identifying PMN prominence in the lung, which occurs in some fibrotic processes.