Background. - Blood C-reactive protein levels have been frequently found to be increased after Curosurf(R) instillation. These variations have been compared to the values after Surfexo(R) therapy and after absence of surfactant therapy. Population and methods. - The files of not infected premature babies, aged 25 to 36 weeks of gestational age, under mechanical ventilation for a hyaline membrane disease (HMD), admitted in our unit between January 1990 to June 1995, have been retrospectively studied. They, were separated into three groups: A: 67 infants ventilated for more than 5 days for HMD without surfactant therapy; B: 23 infants treated by Surfexo(R) C: 60 infants treated by Curosurf(R). CRP was measured daily between day 0 (D0) and D5. Means and standard deviations were calculated for each day and each group. The mean values of CRP at D1 to D5 in group C were compared to D0. The daily CRP values were compared in the three groups. For group C, the results were studied daily according to the gestational age, dosage and age of the neonate at the first instillation. The statistical results have been given according to the Student t test. Results. - After Curosurf(R), the mean CRP value rose significantly from D1 to D4 compared to D0. There was no difference of CRP between groups A and B from D0 to D5. Group C had higher values in comparison to group A (between D1 to D5) and to group B (between D1 and D3). There was no significant difference of the CRP values in group C according to the number of instillations or the amount instilled, but CRP was lower in early treated infants (<H6). Discussion. - Curosurf(R) instillation is followed by a significant increase in CRP, maximum at D2. This is not seen after Surfexo(R). This increase seems less important at D2-D3 when Curosurf(R) is administered early. The CRP increase after Curosurf(R) therapy could be due to an inflammatory reaction to the heterologous proteins it contains.