It has been demonstrated previously that motile Borrelia burgdorferi cells transform into non-motile cyst-forms when incubated for several weeks in BSKII (a complex medium) lacking rabbit serum. B. burgdorferi cells cannot synthesize fatty acids de novo and serum is thought to provide a source of fatty acids and lipids. When B. burgdorferi cells were serum-starved in defined RPMI medium, -90% of the cells formed spherical cysts within 48 h. Cyst formation was inhibited by tetracycline. Cyst opening and recovery of vegetative cells was rapidly induced by the addition of either BSKII or rabbit serum. The percentage of viable cells recovered from cysts ranged from 2.9% to 52-5%. Viability was inversely proportional to cyst age. Protein synthesis by B. burgdorferi during serum starvation was examined by labelling cells with Tran35S-Label and analysing the labelled proteins by two-dimensional gel electrophoresis and fluorography. The synthesis of over 20 proteins was induced during serum starvation. Western blots of proteins from vegetative cells and cysts probed with sera from either B. burgdorferi-infected humans or monkeys revealed that several cyst proteins were antigenic. These data suggest that cells of B. burgdorferi, although possessing a small genome and extremely limited biosynthetic capabilities, rapidly respond to conditions of serum starvation by inducing changes in protein synthesis and cell morphology. This study may help explain how cells of B. burgdorferi can survive periods of nutrient deprivation in different hosts and host tissues.