The present invention describes compositions of thymus derived peptides and uses therefore in diagnostic methods and for the treatment of diseases associated with reduced T helper cell counts, diseases such as infection, e.g., HIV infection and other viral infections, parasitic, and bacterial infection, AIDS, ARC, multiple sclerosis, chronic fatigue syndrome, rheumatoid arthritis, Alzheimer's disease, asthma, allergy, dermatitis, type 1 diabetes mellitus, colitis, inflammatory bowel disease/irritable bowel syndrome, Crohn's disease, Psoriasis, Chronic obstructive pulmonary disease, Systemic lupus erythematosus, transplant rejection and cancer.