The total numbers of mast cells (MC) in popliteal lymph nodes of homozygous nude (nu/nu) and phenotypically 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.