A new tumor marker, tumor-associated trypsin inhibitor (TATI), was studied in 5 patients who received successful kidney or pancreas grafts and in 30 subjects with antibodies against human immunodeficiency virus. Serum TATI concentrations were very high during the four first days after transplantation. Thereafter the serum levels decreased when the peptide was eliminated through the kidney. Consequently, the urine values were very high. The TATI concentrations of HIV positive subjects were compared with serum levels of HIV antigen and antibody, by Western blotting and determination of peripheral T-lymphocyte subpopulations. The occurrence of high concentrations of TATI in some HIV positive subjects and especially in AIDS patients, suggests that TATI could be useful in exploring physiopathological aspects of severe immunodeficiencies even if TATI levels were not correlated with the commonly used markers of the immune system status. The increased levels of TATI in immunological disorders suggests its possible use in assessing the immune response against cancer.