In study 1 we investigated 8 diabetic female patients treated with tolbutamide who received chloramphenicol to combat urinary tract infections. In 5 patients, chloramphenicol was found to produce a distinct decrease of glycemia. Evaluation of the whole series revealed that the latter averaged 31.5%. No patient developed a marked hypoglycemia. With one exception, the values of immunoreactive insulin (IRI) in serum were within normal. In study 2 we investigated another series of 8 diabetic patients. Given chloramphenicol, 7 patients registered an almost twofold increase in the mean morning tolbutamide level in serum and glycemia decreased by one fourth. No patient developed severe hypoglycemia. Tolbutamide increase in serum was not associated with a rise of IRI level in serum. All patients displayed steady chloramphenicol level in serum. Potential reasons for the development of hypoglycemia during the treatment with sulphonylurea antidiabetics are discussed.