Genetic ablation of the murine SP-B gene in transgenic mice caused lethal perinatal respiratory distress in homozygous offspring, whereas heterozygous SP-B (+/-) mice survived postnatally. In adult SP-B(+/-) mice, surfactant protein B mRNA and the alveolar lavage SP-B protein were reduced by 50% compared with wild-type littermates, consistent with the inactivation of a single SP-B allele. Expression of SP-A, SP-C, and SP-D proteins was not affected in SP-B(+/-) mice. Heterozygous SP-B(+/-) mice reached maturity in numbers expected by Mendelian inheritance of a recessive gene. Lung morphology and both intracellular and extracellular phospholipid pool size and composition were unaltered in the SP-B(+/-) mice. Despite normal survival, pulmonary function studies demonstrated a consistent decrease in lung compliance in SP-B(+/-) mice. Abnormalities of inflation/deflation curves demonstrated airway collapse at low deflation pressures. Residual volumes were increased in the SP-B(+/-) mice. In summary, SP-B mRNA and SP-B protein were reduced by 50% in SP-B(+/-) mice, resulting in abnormalities of lung compliance and air trapping, suggesting a potential susceptibility to pulmonary dysfunction associated with SP-B deficiency.