Thymomodulin (Leucotrofina), an orally administrable thymic derivative endowed with interesting immunomodulating and myelomodulating properties, was tested for mutagenicity by means of the following in vitro and in vivo tests: mutagenesis on S. typhimurium with and without metabolic activation system (S-9), gene conversion on S. cerevisiae D 7 with and without metabolic activation, urinary assay in the mouse with and without metabolic activation, urinary assay in the mouse with S. cerevisiae D 7, host mediated assay in the mouse with S. cerevisiae and DNA repair test with and without metabolic activation. On the basis of the results obtained thymomodulin proved to be free of mutagenic activity.