(A) Monocytes were co-cultured with un-treated (Control), solvent-treated (Mph-Solvent) or PMA-activated (Mph-PMA) macrophages and apoptosis was measured 48 h later. Monocytes and macrophages were also concomitantly exposed to PMA (PMA) and apoptosis was measured after 48 h. Exposure of both cell types to PMA exacerbated the killing effect in monocytes. (B) Macrophages themselves showed in the same experimental setting no toxicity after short-term treatment with PMA (Mph-PMA) and only a moderate increase in cell death after direct exposure to PMA (PMA). (C) Monocytes were co-cultured with solvent-treated or PMA-activated macrophages in the absence or presence of the ROS scavenger DMTU (10 mM). In the presence of DMTU, they showed significantly less cell death than monocytes co-cultured with macrophages in the absence of ROS scavenger. Data are the mean of at least four independent experiments ± SEM, 1-way ANOVA, Tukey’s Multiple Comparison Test, *p < 0.05, **p < 0.01, ***p < 0.001.