The cystogenesis event of Toxoplasma gondii is poorly understood. In order to throw light on it, the effect of tumor necrosis factor-alpha (TNF-alpha) was studied in the Prugniaud strain of the organism. This showed that TNF-alpha increased the number of cysts formed in vitro in human MRC5 fibroblasts. The sphingomyelinase pathway may be involved in mediating the TNF effect, since ceramide (natural form in permeabilized cells or cell-permeable analogue) could mimic the action of TNF. More precisely, our results strongly suggest the involvement of an acidic sphingomyelinase in mediating the effect of TNF; indeed, D609 inhibited both the TNF effect and cyst formation, while arachidonic acid had no effect. Moreover, protein kinase (PKC) seems also to play a role in the process, since phorbol-12-myristate-13-acetate (PMA) enhanced the cyst formation. However, chelerythrine chloride did not prevent the TNF effect, suggesting that several host-cell pathways can affect the cystogenesis event. Taken together, these results suggest the active participation of host-cell components in the cystogenesis of Toxoplasma gondii.