Diseases caused by Candida species are an increasing problem. Candida species are associated with high overall mortality, due to a variety of virulence factors such as the yeast-to-hyphal switch and proteolytic enzymes. The phenomenon of microbial communication known as quorum sensing also seems to play an important role. The main characteristics of the quorum-sensing molecule E,E-farnesol are well known for C. albicans. The present study focused on two questions. One of them concerned the secretion of E,E-farnesol by C. albicans and involved a close examination of the effect of the medium (serum) and the origin of the isolates used. The second one dealt with the activity of E,E-farnesol in non-C. albicans species, such as C. tropicalis and C. parapsilosis, e.g. its impact on biofilm formation and growth. Under serum conditions, C. albicans produced up to 58% more E,E-farnesol at 37 degrees C than at 30 degrees C. The growth of all isolates was reduced and delayed by the administration of E,E-farnesol. Of all Candida species, C. tropicalis isolates were most strongly affected by the addition of E,E-farnesol. Biofilm formation on polystyrene was affected by E,E-farnesol treatment in all non-C. albicans species and C. albicans. E,E-farnesol exerts its main effect by altering the metabolic activity and growth inhibition of treated Candida species. The results obtained indicate that the presence of E,E-farnesol in the environment not only regulates the morphology of the Candida species but also affects its fitness. In this regard, the secretion of E,E-farnesol might provide an advantage for members of the microbial community.