The influence of environmental conditions and genotype x environment effects on the alliin content of garlic bulbs were studied. The objective was to improve the quality of garlic bulbs with regard to alliin content. Field trials were carried out for 2 years in France and Spain with increasing sulphur fertilisation in soil: 0-50-100-200 or 400 kg SO4/ha were investigated on 3 varieties in 2000 ('Printanor', 'Messidrôme' and 'Morasol') and only on 'Printanor' in 2001. S fertilisation affected alliin content on 'Printanor' in France, but was not confirmed in Spain where alliin contents were 3 to 5 fold higher than in France. These results were correlated to the soil sulphur in the two areas as Spanish soil was always higher in water soluble and total sulphur than French soil. However, the influence of other factors could not be excluded. Experiments were also carried out in controlled conditions (in vitro and greenhouse) to study the influence of sulphur fertilisation combined to other environmental factors such as light, carbohydrate availability and temperature. A strong genotypic effect was confirmed in both controlled conditions with light spectral quality having a large effect on alliin content on the in vitro bulbs. Temperature did not have a large effect on allying, but did interact with sucrose concentration to affect alliin accumulation in the bulbs. We also found an interaction between S fertilisation and genotype in greenhouse, and genotype by light spectrum interaction, in vitro. Our results confirm that multiple factors affect alliin accumulation in garlic, and that these factors should be considered when growing garlic for flavour or therapeutic value.