Among the discoveries in medicine, vaccines are a major contribution to public health, and they have a preventive role against infectious diseases. The vaccines that contain parts of microorganisms need the addition of adjuvants to activate the immune system. Aluminum adjuvants are often used in vaccines and these cause phagocytic cells to become inflammatory. Examples of phagocytic cells are monocytes that have been differentiated into macrophages. The aluminum adjuvants consist of micrometer-sized particles that are taken up by the cells through phagocytosis, and after phagocytosis, the aluminum adjuvant is in a phagosome. In the phagosome, reactive oxygen radicals (ROS) will be formed to facilitate the degradation of phagocytic material. To investigate whether the presence of ROS is affected due to phagocytosis by aluminum adjuvants, the content of ROS in cells after incubation with aluminum adjuvants was studied. Aluminum adjuvant was tested on THP-1 cell line and THP-1 cell line which were differentiated into macrophages. The presence of ROS has been examined with the MitoSox and CellRox probes and has been detected by flow cytometry. Small differences of content of ROS in the cells were obtained after incubation with aluminum adjuvant. However, the CellRox probe showed mostly a reduction of the content of ROS in the cells after incubation and thus phagocytosis of aluminum adjuvant.