Abstract Background: Fresh frozen plasma (FFP) administration may increase the risk of nosocomial infections in parallel with the development of immune modulation. This could be driven by soluble mediators, possibly influencing the in vitro activation of human U937 monocyte cells, in a manner dependent on the age of the donors. Methods: FFP donors were stratified into groups of 19–30 years, 31–40 years or 41–50 years, and U937 cells were cultured with FFP (alone or plus lipopolysaccharide—LPS) for 24 h. Both in FFP and supernatants, TNF, IL-1β, IL-6, and IL-10 levels were measured by ELISA. Additionally, CD11B, TLR2, and CASP3 gene expression were measured by qtPCR in U937 cells. Total phagocytic activity was also assayed. Results: Elevated IL-10, but low TNF and IL-1β levels were measured in FFP from individuals aged 19–40 years, whereas in individuals aged 41–50 years FFP were characterized by equalized TNF and IL-10 levels. Elevated IL-6 levels were found in all FFP samples, especially in those from the oldest individuals. FFP stimulation was associated with striking modifications in cytokine production in an age-dependent way. Exposure to FFP attenuates the response to LPS. TLR2 and CD11B expression were enhanced regardless of the age of plasma donors, although CASP3 expression was increased only when FFP from individuals aged 19–40 years were tested. Phagocytosis decreased after exposure to FFP regardless of donor age. Conclusion: Our results suggest that soluble mediators in FFP may modulate the functioning of monocytes. Interestingly, this effect appears to be partially influenced by the age of donors.