Native delta-haemolysin of Staphylococcus aureus was poorly immunogenic in the mouse, inducing low anti-delta-haemolysin antibody titres in only 50% of immunised animals. Treatment of delta-haemolysin with formaldehyde at pH 5 or pH 7.5 caused a rapid loss of haemolytic activity and, on treatment for 7 days at pH 5, gave a product of higher immunogenicity than the native material. Treatment of delta-haemolysin with formaldehyde at pH 9.5 for 7 days reduced the haemolytic activity by 97% that failed to enhance the immunogenicity. Antisera to formaldehyde-treated delta-haemolysin contained antibodies directed against antigenic sites not present on the native molecule. Formaldehyde-treated delta-haemolysin had an increased electrophoretic mobility and a reduced isoelectric point but there was no evidence of polymerisation. The enhanced immunogenicity may be due to a reduced affinity for phospholipids, resistance to enzymic digestion, increased rigidity and/or the masking of a suppressor determinant on the molecule. Treatment of delta-haemolysin with glutaraldehyde under acid, neutral or alkaline conditions produced a rapid loss of haemolytic and immunogenic activities. High doses of delta-haemolysin (62.5 - 1000 mug) gave enhanced vascular permeability (EVP) when injected intradermally in the rabbit. The response was obtained at a critical time of 1 h between injection of sample intradermally and of Pontamine Sky Blue Dye intravenously. The EVP activity was resistant to heating at 10
