Pulmonary surfactant is a phospholipid-rich mixture that reduces the surface tension at the alveolar air-liquid interface, providing alveolar stability necessary for normal ventilation. Four distinct proteins which have been isolated from pulmonary surfactant are SP-A, SP-B, SP-C and SP-D. Surfactant precursor protein B (pro-SP-B) with a molecular weight of 42kD undergoes proteolytic processing resulting in a 9kD non-collagenous hydrophobic pulmonary surfactant, SP-B. SP-B mRNA has been detected in both type II cells and in bronchiolar epithelial cells of adult human, mouse, rat and rabbit lung. Pro-SP-B and SP-B mRNA have been detected in approximately 60 per cent and 53 per cent of pulmonary adenocarcinomas, respectively, with expression noted in adenocarcinomas with acinar, papillary, bronchio-alveolar and solid growth patterns. Squamous cell and large cell carcinomas of the lung and nonpulmonary adenocarcinomas do not express pro-SP-B or SP-B. NCL-SPPB is reactive with a proportion of lung adenocarcinomas and is negative with squamous lung carcinomas, mesotheliomas and breast carcinomas.