Pathogens express virulence genes in response to the combination of environmental conditions present in the host environment. The crop is the first gastrointestinal environment encountered in birds. However, feed withdrawal alters the crop environment resulting in an increased pH, and decreased concentrations of lactate, glucose and amino acids compared with unmoulted birds. Salmonella enteritidis infections increase significantly in hens that have been force moulted by feed withdrawal. The present study examined the effects of pH, carbohydrate sources, amino acids and lactate on expression of Salm. enteritidis virulence by measuring expression of hilA. The hilA gene encodes a transcriptional activator that regulates expression of Salmonella virulence genes in response to environmental stimuli. HilA expression was determined using a poultry isolate of Salm. enteritidis carrying a hilA-lacZY transcriptional fusion from Salm. typhimurium. The media used were Luria Bertani (LB) broth and LB broth diluted 1 : 5 (DLB). The expression of hilA was 2.9-fold higher in DLB broth compared with LB broth which suggested that there is a nutritional component to the regulation of hilA. Addition of 0.2% glucose, fructose or mannose to LB and DLB reduced hilA expression 1.5 to twofold. Addition of 0.2% Casaminoacids, arabinose, fucose, or lactose had little effect on hilA expression. Lactate (25 and 50 mmmol l(-1)) reduced hilA expression at pH 6, 5 and 4, with the lowest expression occurring at pH 4. Based on these results it appears that the composition of the crop lumen could potentially influence Saint. enteritidis virulence expression.