Using a monoclonal antibody OKT 6, we nvestigated Langerhans cells by indirect immunoperoxidase technique in 60 skin specimens of fetuses from 3 months of gestation period to 10 months and 21 skin specimens of adults for the study of their development and the comparison of their density in fetus with adult. The results were as follows: 1. Langerhans cell population density of fetus was 52/mm in 4 months of gestation period, l07/mm in 5 months,136/mm in 6 months, l97/mm in 7 months, 235/mm in 8 months, and 311/mm in 10 months and there was a tendency to increase from 4 months to 1Q months. The earliest fetal skin specimen showing OKTg(+) Langerhans cell was that of 13 weeks and 2 days of gestation period. 3. Langerhans cell population density of back skin in fetus was 12.6% in 5 months, 16.0%, in 6 months, 20. 8% in 7 months, 26. 4% in 8 months and 36, 2% in 10 months of 900/mm in adult back skin. 4, Langerhans cell population density in adult was 432/mm in the skin of shin, 363/mm in prepuce, and 90Q/mm in back skin and there was statistically significant in difference between prepuce and back skin(p<0. 00l).