e21603 Background: Despite the efforts, there is still no biomarker that is able to predict the response and / or that can be used in the monitoring of patients treated with check-points inhibitors (ICI). Methods: Prospective study of a panel of blood biomarkers that help predict the response to ICI treatments. Since June 2019, a total of 60 patients with non-small cell lung cancer (NSCLC) or melanoma have been included. Prior to the start of treatment (T1 moment), after the first cycle of ICI (T2), and to progression (Tp) or 6 months after the start, blood sample has been drawn to the patients and the analysis has been carried out by flow cytometry of the percentage of lymphocytes CD8, CD3, Treg, Myeloids, NK and B lymphocytes. The one.way statistical test has been carried out that allows us to see the effect of the response on the variable at each time. Results: Of the 60 patients included, we have radiological assessment in 38. 80% are patients with NSCLC. In the evaluation at 8-10 weeks we found stable disease / partial response in 25 patients (66%), 8 progressions (21%) and 2 unknown reevaluations (13%). A significant increase in CD3, CD8, CD4, and B lymphocytes was observed in responders (p < 0.005); however, an increase in NK was observed in patients in progression (p < 0.02). No significant differences in Treg have been described and the result related to myeloid cells is pending analysis. Conclusions: Our results open a door to peripheral blood monitoring as a new tool for the management of ICI. However, our data are preliminary, having not yet reached the expected sample size and with aspects that we must clarify as the increase in NK in progressors. In addition, the final objective of this study is to generate a score combining several biomarkers of blood and tissue to be used as a predictor of response to ICI (data pending analysis).