Abstract The free living ciliate Tetrahymena thermophila was chosen as a cellular model in order to investigate the mode of action of the anti-inflammatory marine natural product Pseudopterosin A (PsA). In this paper we present evidence that PsA inhibits phagosome formation ( K D = 10.5 μM) and triggers a discrete intracellular calcium release (depletion) from a site in T. thermophila cells ( K D = 6.4 μM). Pre-treatment with the G i/o protein inhibitor, pertussis toxin (PTX), inhibits PsA activity of both responses providing pharmacological evidence that the site of action for PsA is at a PTX sensitive G protein or a G protein coupled receptor (GPCR). Addition of extracellular calcium induced a concentration dependent increase in the incidence of phagosome formation ( K D = 30.3 μM) and was blocked by PsA pre-treatment. This particular effect of PsA on extracellular calcium was not blocked by PTX pre-treatment.