Confocal microscopy was used to image palmitic acid monolayers containing a truncated form of lung surfactant protein B (*SP-B1-25). Analysis of the phase coexistence yielded estimates for the *SP-B1-25 concentration dependent molar area, a. A divergence in the molar area fluctuations is evident at 113 wt% *SP-B1-25. This result suggests that a critical point is located near 11.3 wt% *SP-B1-25 at 22 degreesC and 15 mN/m and that at least three phases coexist below this critical concentration.