The influence of bulk phase status of dimyristoyl phosphatidylcholine (DMPC) on its spreading behaviour at the air-water interface was investigated below and above the main transition temperature (T-m approximate to 24 degrees C). Differently pre-treated samples characterised by differential scanning calorimetry, Karl Fischer titration, thermogravimetry and X-ray studies were placed onto the water surface in a Langmuir trough. Surface pressure as a function of time was recorded by the Wilhelmy plate method. Above T-m spreading pressures of nearly 50 mN m(-1) were obtained for all samples and temperatures. The surface pressure values observed below T-m differed considerably with the spreading source. Thus there is a dependence of spreading behaviour on bulk phase status, e.g. its structure and degree of hydration. Spreading experiments on concentrated NaCl solutions and optical microscopy of the DMPC samples in contact with water confirm the mechanisms proposed for the spreading process. Pre-treatment of DMPC may explain the disagreement in published equilibrium spreading pressure values.