The action of formaldehyde on staphylococcal delta haemolysin

Native staphylococcal delta haemolysin was poorly immunogenic in the mouse. After treatment with formaldehyde at pH 5 or pH 7.5, haemolytic activity was rapidly lost but antigenic reactivity retained and, at pH 5, immunogenicity was enhanced. Treatment of the haemolysin with formaldehyde at pH 9.5 reduced haemolytic activity by 97% but did not enhance immunogenicity. There was no evidence of polymerisation of formaldehyde-treated delta haemolysin, and the enhanced immunogenicity was considered to be partly due to a reduced affinity for phospholipids.