A radiometric microbiologic assay was developed for the determination of niacin in biologic fluids. Lactobacillus planatarum produced 14CO2 from L-[U-14C] h malic acid in quantities proportional to the amount of niacin present. The assay is specific for the biologically active forms of niacin in humans. Normal hemolysates (30) were analyzed and the values ranged from 13.0-17.8 .mu.g niacin/ml RBC [red blood cell] (mean = 15.27 .+-. 1.33 SD). Good recovery and reproducibility studies were obtained with this assay. On 30 blood samples, correlation was excellent between the radiometric and the conventional turbidimetric assays.