Group X secretory phospholipase A(2) (sPLA(2)-X) possesses several structural features characteristic of both group IB and IIA sPLA(2)s (sPLA(2)-IB and -IIA) and is postulated to be involved in inflammatory responses owing to its restricted expression in the spleen and thymus. Here, we report the purification of human recombinant COOH-terminal His-tagged sPLA(2)-X, the preparation of its antibody, and the purification of native sPLA(2)-X. The affinity-purified sPLA(2)-X protein migrated as various molecular species of 13-18 kDa on SDS-polyacrylamide gels, and N-glycosidase F treatment caused shifts to the 13- and 14-kDa bands. NH(2)-terminal amino acid sequencing analysis revealed that the 13-kDa form is a putative mature sPLA(2)-X and the 14-kDa protein possesses a propeptide of 11 amino acid residues attached at the NH(2) termini of the mature protein. Separation with reverse-phase high performance liquid chromatography revealed that N-linked carbohydrates are not required for the enzymatic activity and pro-sPLA(2)-X has a relatively weak potency compared with the mature protein. The mature sPLA(2)-X induced the release of arachidonic acid from phosphatidylcholine more efficiently than other human sPLA(2) groups (IB, IIA, IID, and V) and elicited a prompt and marked release of arachidonic acid from human monocytic THP-1 cells compared with sPLA(2)-IB and -IIA with concomitant production of prostaglandin E(2). A prominent release of arachidonic acid was also observed in sPLA(2)-X-treated human U937 and HL60 cells. Immunohistochemical analysis of human lung preparations revealed its expression in alveolar epithelial cells. These results indicate that human sPLA(2)-X is a unique N-glycosylated sPLA(2) that releases arachidonic acid from human myeloid leukemia cells more efficiently than sPLA(2)-IB and -IIA.