Biophysical activity and inhibition characteristics were studied for a series of synthetic surfactants composed of purified bovine lung surfactant proteins (SP)-B or -C combined with dipalmitoyl phosphatidylcholine (DPPC) and egg-phosphatidylglycerol (PG) in a weight ratio of 80:20:1 DPPC/PG/protein. Surfactant protein preparations included two isolates of SP-B, confirmed by amino terminal sequence analysis, three isolates of SP-C which were free of SP-B by ELISA and Western blot analysis, and two isolates containing both SP-B and -C. In oscillating bubble studies, mixtures of phospholipids with isolates containing pure SP-B or both SP-B and -C lowered surface tension to < 1 mN/m within 2 rain of pulsation at both 1.25 mg/ml and 2.5 mg/ml. Phospholipids combined with isolates of pure SP-C also had substantial activity, lowering surface tension to between 1 mN/m and 3 mN/m depending on the SP-C preparation used. The surface activity of synthetic phospholipid-apoprotein admixtures was decreased significantly by the plasma proteins albumin and fibrinogen, but with varying characteristics depending on the apoprotein preparation involved. At 1.25 mg/ml phospholipid, admixtures containing either isolate of SP-B were inhibited by 10 mg/ml of plasma proteins, as were those with one of the SP-C preparations. However, admixtures containing the two other pure SP-C preparations were not inhibited until 20--50 mg/ml of albumin or fibrinogen were present. The highest resistance to inhibition was shown by admixtures containing one of the apoprotein isolates having both SP-B and -C. These mixtures were not inhibited by up to 50 mg/ml of albumin or fibrinogen, and had surface activity equivalent to an organic solvent extract of natural calf surfactant. Additional experiments showed that the detrimental effects of serum proteins on many of the phospholipid-apoprotein admixtures could be mitigated by raising surfactant concentration, behavior also associated with surfactant extracts and whole lung surfactant. These results are generally encouraging forthe design of synthetic exogenous surfactants by in vitro combination of phospholipids and apoproteins, but the significant variability found in the activity and inhibition of different apoprotein preparations needs to be more fully characterized and understood.