The effects of low concentrations of extracellular ATP on cytosolic Ca²⁺, membrane potential, and transcription of IL-6 were studied in monocyte-derived human macrophages. During inflammation or infection many cells secrete ATP. We show here that application of 10 μM ATP or 10 μM UTP induces oscillations in cytosolic Ca²⁺ with a frequency of ≈12 min^-1 and oscillations in membrane potential. RT-PCR analysis showed expression of P2Y₁, P2Y₂, P2Y₁₁, P2X₁, P2X₄, and P2X₇ receptors, large-conductance (KCNMA1 and KCNMB1–4), and intermediate-conductance (KCNN4) Ca²⁺-activated K⁺ channels. The Ca²⁺oscillations were unchanged after removal of extracellular Ca²⁺, indicating that they were mainly due to movements of Ca²⁺ between intracellular compartments. Comparison of the effects of different nucleotides suggests that the Ca²⁺ oscillations were elicited by activation of P2Y₂ receptors coupled to phospholipase C. Patch–clamp experiments showed that ATP induced a transient depolarization, probably mediated by activation of P2X₄ receptors, followed by membrane potential oscillations due to opening of Ca²⁺-activated K⁺ channels. We also found that 10 μM ATPγS increased transcription of IL-6 ≈40-fold within 2 h. This effect was abolished by blockade of P2Y receptors with 100 μM suramin. Our results suggest that ATP released from inflamed, damaged, or metabolically impaired cells represents a “danger signal” that plays a major role in activating the innate immune system.