The native ferredoxin of Clostridium pasteurianum and its performic acid-oxidized derivative were used as antigens to obtain antisera from rabbits. The antibodies produced were detectable by complement fixation and immune precipitation. Cross-reactivity studies were carried out using these antigens, ferredoxin from which the iron had been removed by trichloroacetic acid (TCA)-precipitation, and ferredoxin that had been reduced and alkylated with iodoacetamide. In all cases considerable degrees of cross-reactivity were observed. The results imply that the cysteinyl residues of C. pasteurianum ferredoxin may not be critically involved as antigenic determinants.