Direct analysis of Δ9-tetrahydrocannabinol (THC) and other cannabinoids in crude acetonitrile extracts of whole blood by liquid chromatography-tandem mass spectrometry using pneumatically assisted electrospray ionization (LC-ESI-MS-MS) was subjected to pronounced ion suppression from co-eluting phospholipids (PLs). The interferences were mainly caused by the lysophosphatidylcholine and lysophosphatidylethanolamine classes of PLs. The PLs were easily removed from crude extracts by filtration through a sorbent with Lewis acid properties, which typically increased the THC and cannabinol (CBN) signal intensities by a factor of 5. Based on this technique, a simple high-throughput LC-MS-MS method was developed for the determination of cannabinoids in 100 μL samples of whole blood. The lower limits of quantification were 0.2 μg/L for THC, CBN, cannabidiol (CBD) and Δ9-tetrahydrocannabinolic acid A (THCA-A) and 0.5 μg/L for 11-hydroxy-Δ9-tetrahydrocannabinol (THC-OH) and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC-COOH). The mean ion suppression levels after clean-up were 10% (THC), 9% (CBN), 17% (CBD), 0% (THC-OH), 2% (THC-COOH) and 9% (THCA-A) at blood concentration levels of 1-10 μg/L. The mean true extraction recoveries were 97% (THC), 101% (CBN), 101% (CBD), 98% (THC-OH), 95% (THC-COOH) and 90% (THCA-A) at the same concentration levels. The relative intra-laboratory reproducibility standard deviations were <9% at concentrations of 1 μg/L or higher. The trueness expressed as the relative bias of the test results was within ±4% at concentrations of 1 μg/L or higher.