The total numbers of masl cells (MC) in popliteal lymph nodes of homozygous nude (nu/nu) and phenolypicaily normal, heterozygous (+/nu) BALB/c mice are greater than in normal (+/+) BALB/c mice at approximately 6 months of age. The values obtained from 4- to 7-month-old (nu/nu) and (+/nu) mice are twice as high as those from 2- to 3-month-old mice. It is concluded that the thymus does not directly determine the number of MC in lymph nodes, but the presence-of the nu gene or another gene closely linked to the nu gene regulates the number of MC.‘Nude’ mice infected with hepatitis virus accumulate many more MC than do healthy mice.