Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are serum-borne lipid mediators with potential proinflammatory and atherogenic properties. We studied the effects of LPA and S1P on [Ca²⁺]_i, a second messenger of cellular activation, in human monocytic Mono Mac 6 (MM6) cells. LPA and S1P induced [Ca²⁺]_i transients with EC₅₀ values of 47 and 340 nM, respectively. Ca²⁺ signals evoked by LPA and S1P originated mainly from the stimulation of Ca²⁺ entry, were blocked by the phospholipase C inhibitor U73122, and were inhibited by pertussis toxin. The LPA₁ and LPA₃ receptor antagonist dioctylglycerol pyrophosphate inhibited the LPA-induced Ca²⁺ signal. Notably, serum and minimally modified LDL (mm-LDL) evoked [Ca²⁺]_i increases that were mediated entirely through activation of LPA receptors. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed the presence of the LPA and S1P receptor subtypes LPA₁, LPA_2, S1P₁, S1P₂, S1P₄ in MM6 cells, human monocytes and macrophages. Together these results indicate that LPA, mm-LDL and serum induce via activation of the LPA₁ receptor a G_i/phospholipase C/Ca²⁺ signalling pathway in monocytes. Our study is the first report showing the receptor-mediated activation of human monocytic cells by low nanomolar concentrations of LPA and S1P, and suggests a role of these lipid mediators in inflammation and atherogenesis.