Steroidogenic acute regulatory-related lipid transfer (START) domains have been found in 15 mammalian proteins, STARD1-D15; the START domain is a lipid binding domain, implicated in lipid metabolism and trafficking and cell signalling. Cholesterol-trafficking START proteins (STARD1, D3, D4, D5) may play a role in macrophage cholesterol homeostasis and, by extension, atherosclerosis, and, by virtue of their lipid binding regions, represent potential drug targets. In this study, we describe qualitative changes in expression of STARD1-D15 during differentiation of human THP-1 monocytes to macrophages, which is accompanied by marked increases in cellular lipid content. Human THP-1 monocytes were differentiated into macrophages, by incubation with phorbol 12-myristate 13-acetate (PMA; 250nM) for up to seven days in RPMI medium containing 10% FBS. Marked changes in cellular phenotype were attended by significant and maximal increases in macrophage phospholipid (5.1-fold, 72.8±16.4μg/4.5×10⁶cells versus 14.3±6.24μg/4.5×10⁶cells; p<0.05, n=4), total cholesterol (3.3-fold, 40.1±7.7μg/4.5×10⁶cells versus 12.3±3.69μg/4.5×10⁶cells; p<0.05; n=4) and triacylglycerol (5.8-fold, 89.7±25.7μg/4.5×10⁶cells versus 15.4±4.96μg/4.5×10⁶cells; p<0.05, n=4) at day 4, compared with monocytic control cells. Using the qualitative polymerase chain reaction (PCR) method and specific primers we found 12 of the 15 known START member genes to be expressed in THP-1 macrophages; expression of STARD6, STARD14 and STARD15 were undetectable using this methodology. Sequence analysis confirmed the expression of STARD1, hitherto thought restricted to steroidogenic tissues, in THP-1 macrophages; the presence of STARD1 precursor protein (37kDa) was also detected by Western blotting. Expression of STARD1 and STARD3 appear up-regulated during macrophage differentiation (0-7days), compared with the housekeeping gene, GAPDH; no apparent changes in the other sterol-trafficking proteins, STARD4 or STARD5 were evident during this process. Examination of the phospholipid trafficking subfamily of START proteins suggested that STARD2, but not STARD7, D10 or D11, was up-regulated during macrophage differentiation, relative to GAPDH. Members of the RhoGAP START family members (STARD8, STARD12 and STARD13) did not appear to be regulated during macrophage differentiation, but, intriguingly, STARD9 was appeared to be induced during this period. Our future studies will investigate the therapeutic potential of key members of this novel family of lipid trafficking proteins by investigating their ability to resolve macrophage lipid accumulation and, potentially, atheroma formation.