A study was made of the blood levels, half-life and elimination of fosfomycin in two groups of underweight infants during the 1st to 3rd days of life and after 3–4 weeks. The test dose was 50 mg/kg i.v., administered in a single dose. The times at which the blood was controlled were 5 min, 1, 3, 6, 9 and 24 h; and 0–24 h for the urine. All samples were evaluated by the microbiological method of diffusion in plate. The results obtained indicate that the elimination of fosfomycin after 24 h is slower and occurs in smaller quantities in the group of infants 1–3 days old than in the group of 3- to 4-week-old infants, the half-life of the antibiotic being 7 and 4.9 h, respect