A series of experiments has been carried out to investigate the adjuvant properties of the amino acid L-tyrosine in laboratory animals. Adsorption of various allergenic materials to L-tyrosine was found to enhance the induction of IgG antibodies, but no unusual propensity to stimulate IgE antibody or delayed hypersensitivity was observed. Administration of the amino acid at a site remote from the allergen was found not to augment antibody production. This, together with evidence of reduced bioavailability of the tyrosine-adsorbed allergen, suggested that the adjuvant activity observed resulted from a short-term depot effect.