The presence of dominant active compounds in standardised methanol extract from the leaves of Stizolophus balsamita (S. balsamita) was examined using HPLC with a diode-array detector. The extract and three dominant parthenolide derivatives were tested with Serotonin Research ELISA for their ability to inhibit the serotonin release from platelets. The antiserotonin effect of the extract was compared with that of parthenolide, a compound with proven antiserotonin and antimigraine effects. This study aimed to evaluate the ability of natural parthenolide derivatives to inhibit serotonin release from platelets. Izospiciformin, stizolin and stizolicin were analysed along with the standardised alcohol extract of S. balsamita leaves, which also contained four other parthenolide derivatives. All the analysed substances were found to inhibit serotonin release from platelets as compared with the control sample, which had 100% of serotonin released. Izospiciformin had the most significant impact (97.98% serotonin release inhibition). The effect of the methanol extract of S. balsamita on the serotonin release inhibition was also statistically significant.