Mixed monolayers of GM 1 glycolipid and stearoyl-oleoyl-phosphatidylcholine (SOPC) or dipalmitoyl-phosphatidycholine (DPPC) phospholipids were studied by surface pressure measurements. The effects induced by GM 1 on the mean molecular areas of mixtures and DPPC phase transition were followed for GM 1 concentrations ranging from 1 to 20 mol.%. Under our experimental conditions, one main parameter influencing the behavior of phospholipid-GM 1 monolayers is the ionic strength of the subphase. Mixed monolayers are in a more expanded state on buffer than on pure water. This could be due to a change of GM 1 orientation at the interface. The interaction of wheat germ agglutinin (WGA), a lectin recognizing specifically GM 1 , with these monolayers was quantified in terms of the Gibbs equation. Specific WGA-GM 1 interactions are clearly reduced in the presence of DPPC as compared with SOPC, probably because of the higher packing density of these monolayers. Phospholipid-GM 1 monolayers could also undergo some rearrangements induced by WGA binding.