Yessotoxin is a generic name for a group of compounds recently discovered and chemically characterized. These compounds pose a classification problem regarding their action, since, although they are considered diarrheic toxins because of their lipophilic nature, their effect is non diarrhogenic. Yessotoxin has been reported to be cardiotoxic, but there is no information about its mechanism of action. We present here preliminary data about the mechanism of action of these toxins in fresh human lymphocytes, as an easily available probe to detect the action of these compounds in humans. The action is mediated by modulation of cAMP and cytosolic calcium levels. Yessotoxins increases cytosolic calcium levels by increasing the release from intracellular reservoirs and the influx of external calcium. The most striking result we have gathered is that yessotoxin inhibits cAMP levels in resting lymphocytes. This effect is very intense, calcium-dependent and seems to be mediated by inhibition of adenylate clyclase or G proteins, an study we are currently performing. This inhibitory effect is not related to the inhibition of protein kinase A or phosphodiesterases. Overall, yessotoxin seems to be a powerful toxin, with a very interesting mechanisms that might be used to study signal transduction.