pl. n. forms of molecules that are identical in chemical composition but differ in the spatial orientation of their atoms (i.e., they are stereoisomers). Enantiomers are stereoisomers that exist in pairs as mirror images. The two enantiomers of a pair rotate the plane of polarized light in opposite directions: L forms produce leftward or counterclockwise rotation (levorotation), whereas D forms produce rightward or clockwise rotation (dextrorotation). In general, L forms tend to have biological activity.