In prophylactic studies in premature and bronchopulmonary dysplasia paediatric populations, the mean half-life of palivizumab was 20 days and monthly intramuscular doses of 15 mg/ kg achieved mean 30 day trough serum active substance concentrations of approximately 40 μ g/ ml after the first injection, approximately 60 μ g/ ml after the second injection, approximately 70 μ g/ ml after the third injection and fourth injection.