OBJECTIVE To determine the broad-spectrum proteinase inhibitor alpha 2-macroglobulin (alpha 2M) and its functional subforms, i.e., free alpha 2M and alpha 2M/proteinase complexes, in human seminal fluid by using specific enzyme immunoassay systems. METHODS The study has been performed in the Andrology Department of the Dermatology Clinics and the Laboratory of Immunopathology of the Institute of Immunology. PATIENTS, PARTICIPATES: Routine patients attending the Andrology Department. METHODS The data have been obtained without particular interventions before or after collection of seminal fluid. METHODS The aim of the study was to determine whether alpha 2M or alpha 2M/proteinase complexes are present in human seminal fluid. RESULTS The concentration of total alpha 2M in human seminal fluid ranged from 1 to greater than 1,000 ng/mL, and between 56% and 85% of the inhibitor was complexed with proteinases. CONCLUSIONS These findings show that alpha 2M and alpha 2M/proteinase complexes have to be considered as functionally relevant biomolecules in male genital tract secretions.