To clarify whether in vitro-cultured basophils have features of mast cells, basophils grown in semisolid and liquid culture were studied ultrastructurally and cytochemically and were compared with freshly isolated basophils and mast cells. Ultrastructurally, the cultured basophils had pleomorphic cytoplasmic granules, some of which were similar to mast cell granules. However, scroll formation, characteristic of mast cell granules, was not observed in cells grown in semisolid and liquid culture. Cytochemically, the reactivity patterns of acidic glycoconjugates in the cultured cells more closely resembled those of the freshly isolated basophils than of the mast cells. In addition, the basophils grown in liquid culture showed a more matured form and had stronger reactivity to glycoconjugates, acid phosphatase and peroxidase than did the cells grown in semisolid culture, suggesting that the liquid culture system was better for the basophil.