125I-Labeled choleragen was bound to liposomes containing galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylce ramide (GM1), but not in large amounts to ganglioside-free liposomes nor to those containing N-acetylneuraminylgalactosylglucosylceramide (GM3), N - acetylgalactosaminyl - (N - acetylneuraminyl) - galactosylglucosylceramide (GM2), or N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N- acetylneuraminyl)-galactosylglucosylceramide (GD1a ). Choleragen released trapped glucose only from GM1-liposomes. This choleragen-induced glucose release from GM1-liposomes was relatively rapid for the first few minutes, then continued more slowly. The amount of glucose released from liposomes in 30 min was dependent on both the GM1content and choleragen concentration. Prior incubation of GM1-liposomes with anti-GM1antiserum prevented the choleragen-dependent release of trapped glucose. After incubation of GM1-liposomes with choleragen, addition of anticholeragen antibodies and complement led to more extensive glucose release. Under these latter conditions a much smaller glucose release was observed also from liposomes containing GM1or N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-ga lactosylglucosylceramide in the absence of choleragen. These releases were attributed to naturally-occurring antiganglioside antibodies in the antiserum and complement. Ganglioside-free liposomes did not release glucose in response to anticholeragen and complement. It appears that choleragen in the absence of other proteins binds specifically to liposomes containing GM1and can induce permeability changes.