Electron microscopic observation of pyocin R1 with the negative staining technique demonstrated that pyocin R1 retained its phage tail-like shape of an extended sheath even when it was inactivated by treatment with p-chloromercuribenzoic acid (PCMB) or 4-(p-sulfophenylazo)-2-mercuriphenol (SAMP). Thus it was shown that the contraction and extension of the sheath does not occur reversibly on the modification of sulfhydryl groups accompanying the change of activity. The activity lost under these conditions was restored to the original level by treatment with 2-mercaptoethanol (2-ME). Numbers of sulfhydryl groups in the pyocin R1 particle were determined to be 208 mol and 152 mol per mol (11.8 x 10(6) daltons) by spectrophotometric titration with SAMP and by membrane-filter assay with radioactive PCMB, respectively. Most of these cysteine residues appeared to be localized in the substructure other than the sheath and core. It was also shown that all of these sulfhydryl groups were not necessary for expression of its activity but a part of them were essential for adsorption to the sensitive cells.