Abstract Cancer is the abnormal growth of cells. Cancer cells multiply rapidly despite space constraints, nutrients shared by other cells, or signals from the body to stop reproduction. Cancer cells are often shaped differently from healthy cells, do not work properly, and can spread to many parts of the body. Tumors are abnormal tissue growth, clumps of cells that can grow and divide uncontrollably. In this article, the anti-proliferative effects of protocatechuic acid (PCA) concentrations determined as a result of cytotoxicity test on C6 rat glioma cells in two different ways were investigated. For this, PCNA immunofluorescence method was used. The effects of protocatechuic acid on C6 cell proliferation were examined using anti-PCNA primary antibody on at least 30 photos from each group. The values of the fluorescent intensity ratios of PCNA-labeled cells between the experimental groups were statistically compared with the control group. Compared to the control group, it was observed that cell proliferation decreased significantly at increasing concentrations both in the groups in which PCA was applied to C6 cells alone and in the groups in the inflammation culture model created with THP-1 human monocyte cells.