BACKGROUND Since its initial discovery in the avian intestine, calbindin-D28K has been reported to occur in various species and tissues. Although calbindin-D28K binds calcium ions in the physiologically relevant range of intracellular calcium, its functional role in the various cell types where it has been localized remains unknown. METHODS We examined the occurrence of calbindin-D28K in the brain and kidney of the testudine reptile, Trachemys scripta, by immunoblotting and immunocytochemistry using rabbit anti-sera directed against rat renal calbindin-D28K and chicken intestinal calbindin-D28K. RESULTS Immunoblotting revealed the presence of calbindin-D28K in the turtle tissues. A single immunoreactive band in the 28,000 relative molecular mass region was visualized in cerebellar and renal homogenates. Immunocytochemistry revealed reaction product for the presence of calbindin-D28K in the Purkinje cells of the cerebellum, and in the distal tubular cells of the nephron. Processes as well as the perikaryon of the Purkinje cell were immunoreactive. CONCLUSION This study describes the occurrence and cellular localization of calbindin-D28K in a reptilian cerebellum, and confirms the phylogenetic distribution of renal calbindin-D28K to the oldest major reptilian group.