The biochemical, immunological, and biological properties of an S layer purified from an Aeromonas hydrophila strain (AH-342) involved in a case of bacteraemia were investigated. The S layer selectively removed from the cell surface was composed of a single acidic (pI 4.56) protein subunit (surface array protein, SAP) with a molecular mass of approximately 52 kDa. Amino acid analysis of this 52 kDa protein indicated a molecule composed of 498 amino acids with 46% hydrophobic residues. No cysteine residues were detected. The first 35 residues of the N-terminus were sequenced by Edman degradation; only 4-24% homology was noted between this sequence and those previously published for SAPs of Aeromonas salmonicida (A450) and a strain of A. hydrophila (TF7) originally isolated from a moribund fish. Polyclonal antibodies raised against AH-342 SAP were genospecific, reacting only against S layers produced by A. hydrophila strains and not those from Aeromonas veronii. Acute serum from the bacteraemic patient from whom AH-342 was isolated reacted strongly with the SAP of AH-342 in immunoblot studies. Purified SAP, when intraperitoneally co-inoculated with SAP- strains of A. hydrophila into Swiss-Webster mice, could reduce the 50% lethal dose by approximately 30-70 fold. The results suggest that the SAP of A. hydrophila strains may play an important role in systemic dissemination after invasion through the gastrointestinal mucosa.