After acute myocardial infarction (Ml), proteolysis of necrotic myocardium is mediated by infiltrating inflammatory cells at the infarct margins. Collagen forms a structural fibroskeleton in healthy myocardium, and after Ml this collagen may continue to provide significant tensile strength to the necrotic muscle wall. To determine whether collagen is also degraded (which might decrease infarct wall strength) and, if so, whether inflammatory cell proteases are implicated, hydroxyproline was measured from infarct zone and normal zone tissue from 24-hour infarcts produced in control rats and in rats made leukopenic (white blood cell count < 300/mm³) by prior whole-body irradiation. Hydroxyproline was measured after precipitation of tissue homogenates with trichloroacetic acid to separate partially degraded collagen from larger collagen molecules that might retain structural importance. At 24 hours, there was significant (25%) collagen degradation in the infarct zone (p < 0.01) in control rats but not in leukopenic rats. Tissue cell counts revealed a paucity of inflammatory cells in the infarct margins in leukopenic rats. Electron microscopic studies revealed greater preservation of collagen in the 24-hour-old Infarcts of irradiated leukopenic rats compared with those of control rats. These results suggest that at 24 hours after experimental Ml in the rat, there is significant collagen degradation mediated by inflammatory cell proteases.