1 Conditions have been worked out for a reliable estimation of the cataleptic activity of delta'-trans-tetrahydrocannabinol (THC) after oral administration to mice, using the ring test over a period of 6 h. 2 By this method, the activity of cannabis herb and 5 crude fractions were measured against THC; at the same time the THC contents were determined chemically. 3 The B/C ratio (biological activity divided by chemical assay) was calculated for each. With cannabis herb the value was 3.3 and with extracts prepared with ethanol or 70% ethanol the values ranged from 3.2 to 7.1, indicating that in all samples the activity was much higher than would be expected from their THC content. 4. The cannabinoids were completely extracted from a sample of herb using petroleum spirit and the marc examined for a possible synergist. Surprisingly, it contained a powerful inhibitor of the action of THC, which could be restored by intraperitoneal prostaglandin E2 (3 microgram/kg). Some crude fractions had inhibitory activities about 10 times that of aspirin. 5 In contrast, the petroleum spirit extract (referred to in 4) had a surprisingly high B/C ratio of 23, indicating that a powerful synergist of THC activity is present. 6 The net effect of the herb and ethanol extracts is probably due to a balance of synergist and inhibitor.