A point mutation in exon 7 of the human surfactant protein B (SP-B) gene, resulting in a cysteine for agrinine substitution (R236C), is associated with partial SP-B deficiency (Ballard et al., Pediatrics 1995). We synthesized, purified, and characterized this mutant SP-B (BR2326C), full length oxidized SP-B1-78(B), and palmitoylated SP-C1-35(C). We compared the in vito function and sensitivity to protein inhibition of BR236C with various synthetic and natural surfactant preparations, including: phospholipids (PL),PL + 3% B (PL-B), PL + 3% BR236C(PL-BR236C), PL + 3% bovine SP-B (PL-Bbov), PL + 3% B + 1% C(PL-B+C), PL + 3% BR236C + 1% C (PL-BR236C+C), and Survanta ®. All surfactant preparations had a minimum surface tension<10 mN/m on a Wilhelmy balance. In the first series of experiments, preterm rabbits born at 27 day gestation were intubated, treated with 100 mg/kg of surfactant, and placed in a temperature-controlled ventilatior-plethysmography systems with 100% oxygen. Dynamic compliance (tidal volume/peak inspiratory pressure) was monitored and pressure-volume curves were performed after 60 min of ventilation. Preterm rabbits treated with PL-B+C, PL-BR236C, and PL-Bbov and lowest values for PL surfactant. In the second series of experiments, the preterm rabbits were treated with surfactant at birth and 2 ml/kg of 1:1 diluted fresh frozen plasma (≈50 mg proteins/kg) intratracheally at 15 min of age. Dynamic compliance decreased sharply after plasma instillation in PL-treated preterm rabbits. Preterm rabbits treated with surfactant preparations containing BR236C maintained the highest dynamic compliance and lung volume after plasma instillation. These experiments demonstrate that the SP-B mutant R236C exhibits in vitro and in vivo surface activity and is relatively insensitive to plasma protein inhibition. The clinical pathology of this SP-B mutant is probably secondary to its inability to reach the alveolar space.