A number of immunological functions has been found to be endowed with biological rhythmicity. Variations of peripheral blood lymphocytes, immediate and delayed hypersensitivity responses, and reactivity of the immune system to antigenic challenge have consistently shown circadian bioperiodicity. By employing a panel of monoclonal antibodies specific for the various lymphocyte subsets, we have been able to detect rhythmic variations in the number of total T lymphocytes, as well as of the T helper (TH) and the T suppressor/cytotoxic (Ts) subsets. Available evidence clearly indicates such circadian variations to be due to compartmentalization of circulating lymphocytes in several lymphoid organs. Biological rhythms (circadian, circaseptan, etc.) have been demonstrated in several immunological situations of clinical interest, including variations of TH and Ts cells in patients with rheumatoid arthritis incidence of allograft rejections, antitumoral effectiveness of chemoimmunotherapy, and allergic symptoms. In addition, the recent availability of synthetic analogues of some thymic peptide hormones and of the synthetic ACTH 1-17 (Synchrodyn 1-17 is greatly contributing to the elucidation of the role of such hormones in the immune regulation process. A preliminary account is reported of the experiments performed with the aim of raising conventional rabbit antisera to the peptide analogue ACTH 1-17, as well as of the consistently negative results which were obtained when 578 human serum samples (from patients treated with such synthetic peptide) were screened by a radioimmunoassay for the occurrence of anti-ACTH 1-17 antibodies. It is emphasized that the recognition of the rhythmic variations of several immunological mechanisms and of the crucial role played by the analogues of adrenal corticotropic and thymic hormones in immune regulation has favoured significant advances in the fast growing area of chronoimmunology.