4-Amino-p-chlorobutyric acid lactam crystallizes as racemic crystals in the monoclinic space groupP21/n, witha=5.006(2),b=6.783(2),c=27.652(2) , =92.25(1),Z=4,R=0.035 for 1618 unique reflections. As both enantiomers are present in a single crystal it is not possible to effect enantiomeric separation by crystal growth inhibition. This lactam may, however, be co-crystallized with (2R,3R)-(+)-tartaric acid forming a 2:1 complex in which only the (R) enantiomer is present. The complex crystallizes in the monoclinic space groupP21 witha=10.074(2),b=10.132(6),c=12.238(6) , =99.13(2),Z=2,R=0.036 for 2291 unique reflections. Complex formation is effected by hydrogen bonding but enantioselectivity does not involve the chiral center of the title compound directly.