Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine that suppresses the production of tumour necrosis factor-alpha (TNF-alpha) by monocytes and macrophages. Suppressor of cytokine signalling-3 (SOCS3), a negative regulator of the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, is induced following IL-10 exposure but recent studies in mice suggest that SOCS3 only targets gp-130-dependent signal transduction pathways. Understanding the signalling pathways responsible for IL-10-mediated effects in primary human monocytes is relevant to human inflammatory disease and necessary for the identification of potential therapeutic targets. An adenoviral transfection system was used to express different levels of SOCS3 (quantified experimentally with its tag green fluorescent protein (GFP)) with the aim of investigating the role of SOCS3 in LPS-induced and IL-10-mediated suppression of TNF-alpha production by non-transformed human monocytes. SOCS3 over-expression had no effect on TNF-alpha mRNA levels induced by LPS or LPS plus IL-10, or on IL-10 phosphorylation of STAT3, STAT1 and ERK1/2. When data from all donors were combined, adenoviral overexpression of SOCS3 significantly reversed the suppressive effects of IL-10 on LPS-induced TNF-alpha production after 2 hr. However, there was a direct correlation between mean GFP intensity (extent of viral infection) and extent of reversal of IL-10's inhibitory effects. Physiological levels of SOCS3 detected in IL-10-exposed human monocytes had no effect on LPS-induced TNF-alpha production. Although overexpression of SOCS3 to supraphysiological levels transiently antagonized the regulatory properties of IL-10 by a post-transcriptional mechanism, these findings suggest that under pathological conditions SOCS3 does not control LPS-activation or the anti-inflammatory properties of IL-10 in primary human monocytes.