Complement activation generates two potent inflammatory mediators from C5, C5a and its derivative C5a(desArg), which results from the removal of the C-terminal arginine by ubiquitous carboxypeptidases. In this paper we describe the purification of milligram amounts of bovine C5a(desArg) by a simplified procedure, and the preparation of mouse monoclonal antibodies (MAbs) to C5a/C5a(desArg) which do not recognize native C5. A MAb was used to develop a sandwich ELISA which made it possible to quantify levels of C5a/C5adesArg in bovine biological fluids. Small amounts (means +/- SEM) of C5a/C5a(desArg) were found in EDTA-plasma (0.58 +/- 0.06 ng.mL-1). The anticoagulant EDTA was more efficient than citrate or heparin in inhibiting in vitro activation of the complement system. Complement activation occurred during coagulation since the baseline concentration of C5a/C5a(desArg) (15.4 +/- 4.1 ng.mL-1) was higher than in plasma. Zymosan, a potent activator of the complement cascade, was used to generate C5a/C5a(desArg). The time-course of the reaction and the dose-effect of zymosan were investigated. Optimal conditions were incubation at 39 degrees C for 1 or 2 h with 2 mg of zymosan per mL of serum. The maximal concentration of C5a/C5a desArg attained in zymosan-activated serum was 4.28 +/- 0.14 micrograms.mL-1. Normal milk (from healthy, uninflamed mammary glands) contained on average 0.12 ng of C5a/C5a(desArg).mL-1 (range 0.02-0.19 ng.mL-1). The maximal amount of C5a/C5a(desArg) which was generated in milk with zymosan was 1.1 ng.mL-1 (range 0.68-2.17 ng.mL-1). In milk from quarters with subclinical infections by coagulase-negative staphylococci, values were 0.18 ng.mL-1 and 2.37 ng.mL-1 for spontaneous and zymosan-generated C5a/C5a(desArg) concentrations, respectively. In milk from Escherichia coli endotoxin-induced mastitis, C5a/C5a(desArg) concentrations (means of four cows) before and after zymosan activation reached 6.5 ng.mL-1 and 55 ng.mL-1, respectively. These results indicate that a C5-convertase can operate in normal milk, that only minute amounts of C5a/C5a(desArg) can be generated (less than 1/1,000 of plasma potential), but that much higher concentrations are reached in milk during endotoxin-induced inflammation. The ELISA made it possible to determine normal ranges of C5a/C5a(desArg) in bovine blood plasma and in milk, and is a valuable tool to define the variations of its concentrations in exudates during inflammatory reactions.