  Studying the supersymmetry enhancement mechanism of Aharony, Bergman,
Jafferis and Maldacena, we find a simple condition on the gauge group
generators for the matter fields. We analyze all possible compact Lie groups
and their representations. The only allowed gauge groups leading to the
manifest N=6 supersymmetry are, up to discrete quotients, SU(n) x U(1), Sp(n) x
U(1), SU(n) x SU(n), and SU(n) x SU(m) x U(1) with possibly additional U(1)'s.
Matter representations are restricted to be the (bi)fundamentals. As a
byproduct we obtain another proof of the complete classification of the three
algebras considered by Bagger and Lambert.
