Prospects for the refinement and reduction of animal usage in the potency testing of Cl.  chauvoei vaccines. 
Protection has been elicited by both cellular and supernatant antigens from Cl.  chauvoei.  The cellular antigen has been solubilised by various treatments described for removing flagella, and has been detected by an ELISA method.  The level of protection elicited by these preparations was poor, and immunoassays using these antigen preparations did not correlate with in vivo results.  The supernatant antigen has been fractionated by ammonium sulphate precipitation and gel filtration.  Using the latter technique, the protective antigen has been partially purified.  Immunoassays using this soluble antigen did not correlate with in vivo protection levels.  It is concluded that in vitro assays based on the partially characterized antigens are not yet suitable for the measurement of protective antibody.