In a study to isolate Clostridium botulinum from field samples, 53 C. botulinum-like isolates were found neutralizable by botulinum antitoxins using the standard mouse bioassay. The neutralization results of 14 of them were found false-positive due to contamination with C. tetani which was separated from these isolates and found neutralizable by the same botulinum antitoxin types. Due to this result, C. tetani was isolated from other five field samples to test their toxins by botulinum antitoxins. They were also found neutralizable by certain botulinum antitoxins. Supernatants of C. tetani isolates were confirmed by being neutralized by tetanus antitoxin. Tetani toxins were found neutralizable by different botulinum antitoxins (A-F) but mainly by polyvalent type ABE (Aventis Behring, Marburg) and type CD mixture (Onderstepoort Veterinary Institute, Onderstepoort). These results were mainly attributed to the non-specificity of the proteins of some (at least ABE and CD) botulinum antitoxin hyperimmune sera which may contain antibodies against C. tetani as well.