At the Anual SLAIP Meeting in 1988 we presented the method of procedure and biochemical features of a Nat obtained by bronchioalveolar lavage of bovine lungs. This product is how being elaborated by an Argentinan laboratory. Between 8-1-90 and 7-31-91, 40 premature infants (Pret) less than 37 weeks gestational age, diagnosed a HMD and in mechanical ventilation, were treated by tracheal insulation with Nat (90mg/kg) in the first hours of life. Additional doses were given up to 48 hs., based on clinical requirements. The treated group had a birth weight of [xmacr ]=1268gr (r=630-2470) and a gestational age of [xmacr ]=29.1 weeks (r=23-36). The first dose was administered at [xmacr ]=4.7hs. (r=0.5-24). Before it the FiO2 was [xmacr ]=0.81 (SD=0.2), the PaO2 [xmacr ]=59mmhg (SD=15) and the mean airway pressure (Paw) [xmacr ]=9.2cm H2O (SD=3.8). Ten minutes afterwards the PaO2 was [xmacr ]=145mmHg (SD=51)(p < 0.001). One hour late the FiO2 was [xmacr ]=0.54 (SD=0.28) (p < 0.001) and the difference was maintained during the first 72 hours. The Raw decreased significantly only after 12 hs [xmacr ] Paw=7,4cm H2O DS=3.2 p=0.02). Each patient received a mean of 2.1 doses. Sixteen Pret (40%) required only one dose, 16 (40%) received a second dose (age [xmacr ]=8.7hs r=2.5-2.4) and the remaining 8 (20%) three or four doses. From 17 Pret with birth weights < 1000gr., 10 died (58%), 7 from causes associated with the HMD There were no deaths in the 23 Pret with birthweights > 1000gr. Seven patients (17.5%) had air leaks (4 pneumothorax and 4 interstitial emphysema). Three Pret developed bronchopulmonary dysplasia. Nat may be helpful in the treatment of the HMD in Pret.