A new lactam has been isolated from Jarropha macrorhiiu (Euphorbiaceae) and has shown tumor-inhibitory properties against the P-388 lymphocytic leukemia test system. The compound was identified by means of NMR, UV, IR, rotary dispersion, mass spectrometry, and classical chemical procedures. It has the structure 5-hydroxy-4-methyl-3-pyrrolin-2-one and has been named jatropham.