There has been a renewed interest in the use of the metal-on-metal (MOM) implants for total hip arthroplasty (THA). It is well known, however, that the MOM articulation generates both metal particles and ions. The physiologic effects of these ions are poorly understood and their potential toxicity remains a cause for concern. In the present study, murine J774 macrophages were incubated with Co2+ and Cr3+ ions and the mode of cell death (apoptosis/necrosis) was evaluated in vitro by transmission electron microscopy and cell death ELISA. Overall, results demonstrated that the mode of cell death was dependent on the ion concentration and the incubation time. Indeed, at short incubation times (24 h), the noninflammatory process of apoptosis was predominant. At longer incubation times (48 h), however, necrosis was predominant at higher ion concentrations.