Three thyroid stimulators; LATS, TSH, and cholera toxin, have been tested by bioassay using congenitally athymic homozygous mice (nu/nu), nude mice and control littermates (nu/+) possessing normal thymic function. nu/+ Animals showed a response to LATS similarly to normal white mice of the ddN strain. The assay response to LATS in nu/nu animals was about half that in normal mice. TSH also showed a less stimulating action in nude mice. Cholera toxin made a long acting response similarly to LATS. The response to cholera toxin in nude mice was about half that of normal control mice. The thyroid of nude mice has a receptor for LATS-immunoglobulin, TSH or cholera toxin, which is similar to that of normal mice.