Food and feed products made of cereal grain can be contaminated by mycotoxins that can adversely affect the health of animals and humans. To determine how human consumers are affected it is important to explore the effects of mycotoxins on human cells. The overall aim of the present study was to investigate if low concentrations of mycotoxins had any effects on monocytes, on the differentiation process from monocyte to macrophage and on macrophage functionality. The effect of six mycotoxins: 2-amino-14,16-dimethyloctadecan-3-ol (AOD), alternariol (AOH), deoxynivalenol (DON), enniatin B (ENNB), sterigmatocystin (ST) and zearalenone (ZEA) on the number of viable cells, necrosis, apoptosis and the cell cycle in the human monocyte cell line THP-1 has been investigated. All of the mycotoxins reduced the number of viable cells with the relative potencies: DON>ENNB>AOD>AOH>ST>ZEA. At the higher concentrations tested, all of the toxins caused cell death by necrosis and the relative potency was: DON>ENNB>ST>AOD>ZEA>AOH. Furthermore, all the toxins, except ZEA, also increased the amount of apoptotic cells, with DON and ENNB being the most potent. AOH, ST and ZEA led to a clear cell cycle arrest in the S- and G2/M-phase, S-phase and G2/M-phase, respectively. To characterize the differentiation process from monocyte to macrophage cell morphology, expression of cell surface markers (CD11b, CD14 and CD71) and cytokine secretion of TNF-α were investigated. None of the toxins, tested at non-cytotoxic concentrations, affected the differentiation process as judged by cell morphology. However, AOH, DON and ZEA inhibited the expression of specific cell surface markers. These three toxins also affected macrophage functionality. DON enhanced the lipopolysaccharide (LPS)-induced secretion of TNF-α, while AOH and ZEA reduced the secretion. Combined effects of AOH, DON and ZEA on the CD14-expression were explored by applying the concentration addition (CA) and independent action (IA) prediction models, revealing additive effects. Altogether, the six mycotoxins affected the THP-1 monocytes differently, both qualitatively and quantitatively. AOH, DON and ZEA also had adverse effects on the differentiation process from monocyte to macrophage and altered macrophage cytokine secretion.