Guinea pig bone marrow basophils were cultivated in vitro and morphological observations were made by light and electron microscopy. Maturation of basophils in vitro is supported by electron microscopic evidence for the development of a prominent Golgi zone with granule formation, nuclear chromatin changes, and specific periodicity of the intragranular matrix. Histochemical staining distinguishes these cells from other granulocytes, and mitotic basophils are recognizable on light microscopy. These methods allow more detailed study of the orderly growth and development of the basophil in a system where basophilopoiesis is actively taking place.