SUMMARY:Certain aspects of the procedure for the nutritional classification of soil bacteria developed in this laboratory are discussed, particularly in the light of the criticism by Taylor (1951a). Though any system of nutritional grouping is admittedly susceptible to modifications as advances are made in our knowledge of bacterial nutrition, Taylor's criticism reveals, in part, a lack of understanding of the principles underlying various procedures; consequently certain objections are pointless. Possible merit in other points of objection is obscured by a lack of experimental data.Taylor's proposed scheme of classification indicates a different approach, based as it is on a study of cultures isolated from a more selective medium than ours, which latter was designed as being the least selective and one most favouring the ‘autochthonous’ soil bacteria. Furthermore, his method for assessing growth response in the differential liquid media appears to differ radically from ours. For the purpose for which our system of nutritional grouping was designed such changes are considered disadvantageous.Though Taylor is in agreement as to the need of certain strains of bacteria for some factor or factors present in soil extract, his attributing of a growth factor effect to the presence of calcium is open to objection in the absence of this element in the basal medium; the significance of the interaction between calcium and various peptones must now be interpreted in the light of the discovery of the role of vitamin B12in the nutrition of many soil bacte