Thymic hormones are required for maturation and maintenance of the immune efficiency. It has been previously demonstrated that with advancing age there occurs a progressive reduction of the plasma level of one of the best known thymic peptides, i.e. thymulin, and that the administration of an amino acid combination (lysine-arginine, as present in the commercial preparation Lysargin, Baldacci, Italy) to elderly individuals is able to increase the synthesis and/or release of thymulin to values comparable to those recorded in young subjects. In the present paper we report evidence that cancer patients show much lower thymulin values than those recorded in healthy age-matched individuals and that the oral administration of the amino acid preparation is able to significantly increase thymulin levels even over the values of age-matched controls and to increase the number of peripheral T-cell subsets. It is suggested that such an effect is mediated through the known secretagogue activity of the amino acids on the pituitary release of growth hormone, which has a modulating effect on the thymic endocrine activity.