ABSTRACT: Serum from seven of 15 patients with non-seminomatous testicular germ cell tumors impaired the transformation of lymphocytes from normal donors by phytohemagglutinin to less than 50% of that with addition to the cultures of normal AB serum. Similarly, serum from eight patients impaired the transformation by Staphylococcus aureus Cowan I. The impairment of the lymphocyte transformation did not correlate with the serum concentrations of alpha-fetoprotein (S-AFP) and human chorionic gonadotropin (S-HCG). The patients with metastases and serum that impaired lymphocyte transformation to less than 50% of that With addition of serum from the controls and the other patients survived in the same way (0.50 < p < 0.75, logrank test). The prognosis of patients with testicular germ cell tumors seems not to be influenced by S-AFP and S-HCG through an impact on lymphocyte functions determined as the response to the two mitogens.