The fate of Salmonella enteritidis PT4 in home-made mayonnaise prepared with citric acid solution (citric acid concentration of Ð4·98% (w/v)) was investigated. It was found that pH of mayonnaise is closely related to the ratio of egg yolk to citric acid, and the inactivation rate of the micro-organisms increases as the ratio decreases and/or incubation temperature increases. To achieve Salm. enteritidis PT4-free home-made mayonnaise prepared with pure lemon juice (citric acid concentration Ð5% (w/v)), it is recommended that the pH should be 3·30 or below, or, in practice, at least 20 ml pure lemon juice per fresh egg yolk should be used. For the use of 20–35 ml pure lemon juice per egg yolk, the product should be held at 22 °C or over for at least 72 h and for the use of over 35 ml pure lemon juice per egg yolk, for at least 48 h before consumption or refrigeration.