The functional activity and antigenic expression of the fifth component of complement (C5) in concentrated bronchoalveolar lavage fluids (BALF) and serum samples obtained from 13 nonsmoking and 13 smoking baboons was quantified by specific hemolytic titration and radioimmunoassays. The number of C5H505 units per milligram of albumin in BALF obtained from nonsmoking and smoking baboons indicated a nearly 2-fold increase in C5 hemolytic activity in lavage fluids as compared to matched serum samples. No significant differences (p greater than 0.05) in C5 hemolytic or antigenic (radioimmunoassay) expression were evident between BALF samples obtained from nonsmoking and smoking baboons. C5 stability in BALF was indicated by the highly significant correlation (p less than 0.005) between C5 hemolytic activity and antigenic expression. The absence of C5-cleaving enzymes in BALF was verified by incubation of 125I-C5 with these fluids, followed by SDS5 polyacrylamide slab gel and autoradiographic analyses. These results demonstrated the presence and stability of substantial quantities of functionally active C5 in the secretions of primate lungs, which represents a readily available source of phlogistic fragments for the initiation of pulmonary inflammation.