The hypothesis that non-specific histopathological gut changes are thymus-dependent was tested in a helminth infection--the nematode Trichinella spiralis. The study was performed in congenitally athymic (nu/nu) mice and their thymus-bearing heterozygous litter-mates (+/mu). The non-specific histopathological changes of the small intestine were judged on villus/crypt ratio and mitotic index; for the specific (= immunological) reaction the number of pyroninophilic cells in the small intestine was used as criterion. T. spiralis induced non-specific histopathological reactions both in nu/nu and +/nu mice, whereas the number of pyroninophilic cells was only increased in +/nu mice. It was concluded that the immunological reaction was dependent on the immune status of the host, whereas the non-specific histopathological changes were not, i.e. they were thymus-independent.