20 patients with primarily inoperable squamous cell carcinoma of the head and neck were treated with 2 cycles of chemotherapy prior to surgical treatment. The lymphocyte subsets were determined prior to chemotherapy, as well as directly after and 3 weeks after start of each chemotherapy cycle. For this, 2 different flow cytometric techniques were used in parallel. After the second cycle of chemotherapy 10 patients were additionally treated with thymostimulin for 2 weeks. The remaining ten patients received no additional immunotherapy and served as control group. Three patients (one in the control group and two in the verum group) had to be excluded from the study because the immunological examination could not be performed in accordance with the protocol. In addition another patient had a local skin reaction so that thymostimulin therapy could not be started as allergy to bovine proteins was suspected. Prior to the first chemotherapy treatment, there is no detectable defect in the distribution of lymphocyte subsets. Under chemotherapy treatment the absolute number of Pan-T cells, T-helper cells, T-suppressor cells and Leu-10 positive B-cells decreased. In contrast, additional treatment with thymostimulin directly after the chemotherapeutic regimen resulted in a marked increase of all T-lymphocytes as well as the Leu-10-positive cells. The results measured with two different flow cytometric techniques were comparable.