Adsorption of glucose oxidase (GOx) at the solution/air interface in the presence and in the absence of spread monolayers of dibehenoylphosphatidylcholine (DBPC) has been monitored by surface tension and surface potential measurements. At the monolayer-free interface and at the solution concentration levels of the enzyme higher than 4 mu g/mL, this adsorption was found to be concentration and time dependent. Below this threshold concentration value, no decrease in the surface tension could be observed. Conversely, in the presence of dibehenoylphosphatidylcholine (DBPC) monolayers, increments in surface pressure were recorded even at GOx solution concentrations at which no diminution of the surface tension at the monolayer-free interface was observed. These increments in surface pressure decreased with the increase in the initial surface pressure of DBPC monolayers independently of the mode of compression of monolayers (addition of DBPC aliquots at constant area or dynamic compression). The increase in surface pressure following the injection of GOx molecules beneath a DBPC monolayer has been attributed to the occurrence of a hydrophobic interaction between hydrocarbon chains of the phospholipid and the enzyme. The differences in the magnitude and in the rate of the enzyme penetration into spread DBPC monolayers are discussed and modeled with respect to the mode of compression of monolayers.