Cantharidin and its analogues have been of considerable interest as potent inhibitors of the serine/threonine protein phosphatases 1 and 2A (PP1 and PP2A). However, limited modifications to the parent compounds is tolerated. As part of an on-going study we have developed a new series of cantharidin analogues, the cantharimides. Inhibition studies indicate that cantharimides possessing a D- or L-histidine, are more potent inhibitors of PP1 and PP2A (PP1 IC₅₀=3.22±0.7 μM; PP2A IC₅₀=0.81±0.1 μM and PP1 IC₅₀=2.82±0.6 μM; PP2A IC₅₀=1.35±0.3 μM, respectively) than norcantharidin (PP1 IC₅₀=5.31±0.76 μM; PP2A IC₅₀=2.9±1.04 μM) and essentially equipotent with cantharidin (PP1 IC₅₀=3.6±0.42 μM; PP2A IC₅₀=0.36±0.08 μM). Cantharimides with non-polar or acidic amino acid residues are only poor inhibitors of PP1 and PP2A.