SUMMARY: The content of ribonucleic acid (RNA), excess organic phosphate (XSP) in the RNA fraction, deoxyribonucleic acid (DNA), acid-soluble inorganic phosphate (AI) and organic phosphate (AO), and the lipid phosphate (LP) and lipid weight (L) have been determined in some ten Gram-positive and ten Gram-negative organisms, grown in the same media and harvested at the same phase of growth. A statistical analysis of the results shows that the RNA and DNA contents of the Gram-positive and Gram-negative groups do not differ significantly; the percentage of lipid phosphorus is only half as great in the Gram-positive as in the Gram-negative group; and XSP is present in all the Gram-positive organisms, but absent from all the Gram-negative ones. We suggest that a lipid-polyglycerophosphate-protein component may be a common structural feature of the cell envelopes of all Gram-positive organisms. The mechanism of the Gram reaction is considered in the light of our observations.