A dry syrup preparation for infants and children of a newly developed 16-membered macrolide antibiotic, rokitamycin, was administered to 5 neonates and low birth weight infants of 6 to 25 days after births at a dose level of 10 mg/kg on an empty stomach then plasma drug levels were determined. The dry syrup preparation was also given to a total of 19 Chlamydia trachomatis infection cases of 7 days to 8 months old neonates, low birth weight infants and infants including 12 cases of pneumonia, 2 cases of conjunctivitis and 5 non-symptomatic carriers at an average daily dose level of 48.1 mg/kg in 2 to 4 doses for an average of 19 days and its clinical effects, bacteriological effectiveness, side effects and effects on laboratory test values were examined. The obtained results are summarized as follows. 1. Because the test subjects were neonates and premature infants, obtainable amounts of blood samples were limited, thus it was not possible to determine time courses of plasma drug levels to reach their peaks. Peak plasma levels, however, were speculated to be similar to those in children. Plasma half-lives of the drug were also not determinable, but they seemed to be somewhat longer than those in children. 2. Clinical efficacies were determinable in the 2 cases of conjunctivitis and 10 of the 12 cases of pneumonia, with excellent or good results in both cases of the former and with excellent or good results in 9 of the 10 determinable cases of the latter. Thus, the overall efficacy rate was high, 91.7%. 3. Bacteriological efficacies were determinable in 18 cases including non-symptomatic carriers. C. trachomatis was eradicated in 16 of the cases with an overall efficacy rate of 88.9%. 4. Diarrhea was observed in 2 cases, which were suspected as side effects of the drug. 5. No abnormalities were observed in the laboratory test results. Judging from the above results, this drug appears to be useful for the treatment of C. trachomatis infections of neonates, low birth weight infants and infants.