The morphology and cytochemistry of the blood basophil of Bufo marinus are characterized, and compared and contrasted to the basophil from several other amphibians and with the mammalian basophil. The basophil of B. marinus contains at least one type of mucopolysaccharide and an acid mu- copolysaccharide, possibly heparin, within the cytoplasmic granules. Several amino acids, but no histidine, and no lipid, occur in B. marinus basophils. Furthermore, these basophils appear devoid of hydrolytic enzymes, excepting nonspecific esterase. They do possess numerous oxidative enzymes, however, in the intergranular cytoplasm. The basophil of B. marinus is more similar cytochemically to the mammalian basophil than to the basophil of several previously studied amphibians. Considerable information regarding the morphology and cytochemistry of the mammalian blood basophil is available (Wetzel et al., 1967; Ackerman and Clark, 1971; Komiyama and Spicer, 1974; van El- ven et al., 1977). Ackerman (1963a) pre- sents an excellent review and comparison of basophil cytochemistry in several mam- malian species, including man; he also compares and contrasts the basophil with the mast cell. In contrast, although discov- ered over a century ago, the precise func- tions of the mammalian basophil remain obscure, despite its involvement to some extent in phagocytosis, allergic reactions, and delayed hypersensitivity (Stossel, 1977). Even less is known about the am- phibian basophil. Due to the fragility of the basophil, and its infrequency (1% or less) in the peripheral blood (factors which also hindered study of this cell in mam- mals) few data regarding the amphibian basophil are available. Caxton-Martins (1978) cytochemically examined the leu- kocytes in two West African anurans, Rana temporaris and Bufo regularis, but reported his findings as granulocytes, not differ- entiating among neutrophils, eosinophils, and basophils. In the newt, Notophthalmus viridescens, Cowden et al. (1964) demon- strated the presence of mucopolysaccha- ride and tyrosine in basophilic leukocytes and tissue mast cells.