It has been shown that the growth rate of L. casei differs in PGA and serum enriched media. Since the serum folic acid level is derived from a comparison of the optical densities in these media it follows that this comparison will yield changing results until growth has stopped due to exhaustion of the available folate. If the assay is terminated before this stage is reached, then nonreproducible results can be easily obtained. Variations in the number of viable organisms in the inocula from assay to assay (which are not easily avoidable) accentuate the possible error. Both these problems appear to be controlled if the usual incubation period is extended to the stage at which growth of the organism has ceased. Our normal range under these conditions is 4.5 ng./ml. to 12.4 ng./ml. with a mean of 7.1 ng./ml.