Objective of the research was determination level of RRM1 expression in ovarian cancer tissue. Materials and methods. To avoid disadvantages of those evaluations, the present study of the RRM1 expression level in ovarian cancer tissue determination was held using strictly quantitative immunofluorescence flow cytometry-associated method developed by the authors. Results. RRM1 expression was detected in 100 % of samples of ovarian serous adenocarcinoma, with significant differences in the indicator level in variable patients. The high level of RRM1 (≥ 40 % of cells expressing the marker) was detected in 65 % of the tumor samples studied, low (< 40 %) - in 35 % of cases. Data on the lower expression of RRM1 in one-third of the test samples from primary serous ovarian cancer corresponds with clinical observations of gemcitabine monotherapy efficiency in 25-30 % of patients. Conclusion. The authors believe that a strictly quantitative determination of the marker level in tumor tissue can help to overcome the ambiguity of RRM1 clinical significance estimation in gemcitabine treatment effectiveness prediction.