The blue-green algal toxin cylindrospermopsin (CYN) inhibits protein synthesis, and CYP450 enzymes metabolise CYN to cytotoxic endproducts. Human chorionic gonadotrophin (hCG) stimulates the de novo synthesis of StAR and CYP450 aromatase. Human IVF-derived granulosa cells (GC) (n=7) were exposed to 0-5μM CYN±1IU/ml hCG for 2-24h. After 24h pre-culture GC responded to hCG by increasing estradiol 17β (E 2) and progesterone (P 4) synthesis. Three micromolar of CYN±1IU/ml hCG for 24h was not cytotoxic and did not affect basal or hCG-stimulated E 2 or P 4 production, but did inhibit protein synthesis (p<0.05, n=4). hCG-stimulated steroidogenesis was not reduced by CYN, suggesting a lack of effect on StAR or CYP450 aromatase protein synthesis. hCG enhanced the effects of CYN on GC protein synthesis. Twenty four hours exposure to 0.1μM CYN did not affect GC, supporting the establishment of a 0.0024μM Guideline level for CYN in public water supplies.