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A Xenopus-specific anti-leukocyte monoclonal antibody designated XL-2 was isolated and used to identify leukocytes in tailbud embryos and activin A-treated explants of blastula animal cap. XL-2 bound to a 135-kDa polypeptide in western blots of protein extracts from adult thymocytes, tailbud embryos, tadpoles, and explants. In cell suspensions, it immunostained the cell surface of all types of adult leukocytes including lymphocytes, monocyte/macrophages, thrombocytes, and granulocytes. At embryonic stage 24, immunocytochemistry revealed XL-2-positive leukocytes, the earliest time at which such cells have been recognized. Whole-mount staining of tailbud embryos and tadpoles showed a widely dispersed population of XL-2-reactive leukocytes, many of which had elongated shapes and ameboid pseudopodia. In activin A-treated animal caps, XL-2 recognized a subpopulation of cells within the lumen of the central fluid-filled cavity as well as cells in the interstitium of mesenchymal and mesothelial components of the explant. Together, activin A and human interleukin-11 induced 100% of explants to form lumenal blood cells. Compared to activin A alone, murine stem cell factor plus activin A significantly increased the numbers of XL-2-reactive leukocytes and erythrocytes. These results support the view that activin A induces leukocyte and erythrocyte progenitors during Xenopus embryogenesis.