A radioimmunologic assay method was employed for assessing the hematic and urinary levels of beta methyl-digoxin in patients with various degrees of renal function, evaluated by creatinine clearance test. Significantly elevated hematic values and reduced clearance rates of MD were observed only in patients whose creatinine clearance values were below 50 cc/min. A good correlation between values of creatinine and MD clearances and between their ratio and serum albumin levels was verified. The authors stress the usefulness of this ratio as an indirect index for the plasmatic percentage of the free and of the protein- bound fractions of MD.