Autolytic activity in the soluble and sediment fractions of sonicates of the spiral and the coccoid form ofCampylobacter upsaliensiscould not be demonstrated by native (nondenaturing) polyacrylamide gel electrophoresis (PAGE). Autolysins were detected, however, by using denaturing sodium dodecyl sulfate (SDS) - PAGE gels containing either purifiedEscherichia colipeptidoglycan or whole cells ofMicrococcus luteus(Micrococcus lysodeikticus) as the turbid substrate, with subsequent renaturation by treatment with Triton X-100 buffer. In renaturing gels that containedEscherichia colipeptidoglycan, 14 putative autolytic bands ranging from 200 to 12 kDa were detected. In similar gels containing whole cells ofM.luteus,only a single band appeared with a molecular mass of 34 kDa. This band corresponded to one of the bands present in the gels containingEscherichia colipeptidoglycan. This common autolysin was isolated by adsorbing it fromCampylobacter upsaliensissoluble fractions ontoM.luteuscells and then subjecting these cells to renaturing SDS-PAGE in gels containingEscherichia colipeptidoglycan. The 34-kDa autolysin differed from a single 51-kDa autolysin unique to theM.luteuscells, and when isolated from an SDS-PAGE gel, was pure when tested by isoelectric focusing. The N-terminal amino acid sequence analysis showed the first 15 amino acids of the 34-kDa autolysin to have 67% identity to a part of antigenic protein PEB4 ofCampylobacter jejuni. The purified autolysin was used to immunize rabbits and the antibodies produced precipitated autolytic activity from cell lysates. The specificity of the antibodies was shown by Western blotting: only a single specific band occurred, with a molecular mass of 34 kDa, and thus it seems unlikely that the 34-kDa autolysin was derived from any of the other autolysins that were detected.Key words: autolysin,Campylobacter upsaliensis, zymogram, murein hydrolase.