e20018 Background: The purpose of the study was to evaluate parameters of cellular immunity in lung cancer patients with various levels of circulating tumor cells (CTCs) and different tumor histogenesis. Methods: Blood samples of 28 patients (24 men, 4 women, mean age 62±1.3 years) were studied. 18 (64.3%) patients had neuroendocrine tumors (small cell lung cancer, SCLC), and 10 (35.7%) patients had adenocarcinoma (AC). 11 (39.3%) patients were diagnosed with stage III disease and 17 (60.7%) – stage IV. CTCs were detected in the blood before treatment using the CellSearch System (Janssen Diagnostics, LLC) with iron microparticles coated with antibodies to EpCAM, CD45 and cytokeratins 8, 18 and 19; subsets of lymphocytes, including minor ones, were studied on FACSCantoII flow cytometer (BD). Results: CTCs were found in 25 patients (89.3%); their number ranged 1-6888. 15 (83.3%) patients with SCLC had 0-6888 CTCs, with an average of 655.7, median 38.5. All 10 patients with AC had 1-486 CTCs, with an average of 57.2, median 5.0. The number of CTCs in CTC-positive patients with SCLC was, despite high individual variability, significantly higher than in patients with AC (786.9±474 vs. 57.2±47.9, respectively; p < 0.05). Such differences were accompanied by higher levels of Th-lymphocytes with activation/apoptosis markers (CD3+CD4+CD95+): 84.2±2.7 vs. 72.2±3.7%, and cytotoxic CD16bright/56dim NKcells: 91.4±1.8 vs. 83.4±3.7%. Patients with CTC+ AC had higher levels of IFNγ-producing NK-cells, compared to SCLC patients (15.3±3.8 vs. 7.2±1.6%), as well as levels of Th with the early activation marker (CD4+CD38+) (27.8±3.5 vs. 17.4±2.3%). Presence of CTCs in SCLC patients was associated with lower levels of activated lymphocytes of various subsets: T-cells (CD4+CD38+, CD8+CD38+, CD3+CD8+HLA-DR+) and NК-cells (Granzyme B+ and CD335+). Conclusions: Levels of CTCs were significantly higher in patients with SCLC than in those with AC; immune status parameters had a number of differences in CTC+ tumors with various histogenesis. The presence of CTCs in SCLC was accompanied by reduced blood levels of activated T- and NK-cells. The reported study was partially supported by RFBR, research project No. 16-34-00244mol_a