Cryptococcosis in nude (nu/nu) mice was examined histopathologically. In addition, effects of carrageenan and lymph node cell transfer against Cryptococcus infection were investigated. As controls, heterozygous littermates (nu/+mice) and mice of strain ddy (ddy mice) were employed. Each mouse was inoculated intravenously with 10(5) yeast cells of Cryptococcus neoformans suspended in 0.2 ml phosphate-buffered saline. Two mice out of each group were sacrificed at appropriate intervals up to 25 days after inoculation and histopathological sections were prepared from them. They were stained with H & E and by PAS method. Histopathological characteristic in the brain was cyst formation with no cellular response. The brain was more severely in the nu/nu mice than in either the nu/+ ddy mice. In the liver, there was a major difference in histopathological findings between the nu/nu and either of the other groups of mouse. In the nu/nu mice, cyst formation with no cellular response was induced, and on the contrary granuloma formation in the nu/+ and ddy mice. However, the granuloma formation was inhibited in the livers of the nu/+ and ddy mice by administration of carrageenan, and induced in the nu/nu mice be cell transfer. In the spleen and lymph nodes, lesions were severer in nu/nu mice than in either the nu/+ or ddy mice. These results suggested that the fungus' invasiveness of mice was strongly influenced by T-cell dependent mechanism.