The histochemistry of masked lipids of chromosomes has been investigated with the aid of paper chromatographic procedures. At least three tightly bound associations of phospholipid with protein have been identified in calf thymus nucleohistone. Hence it seems likely that the ‘spurious’ reactions of nuclei and of nucleohistone, especially after extraction with lipid solvents, is a true indication of the increased availability of these closely linked phospholipids. Moreover, not more than about 10% of the material extracted by hot pyridine from calf thymus was fatty, the rest being watersoluble. After treatment of plant cells with hot trichloroacetic acid had produced increased staining with methods for demonstrating lipids, fatty matter could be extracted from the tissues. This demonstrates that the cells which gave the ‘spurious’ reaction did contain lipid. Hence it seems probable that extraction with solvents does not remove all lipids but may make those that remain more available for staining, so giving rise to what has been considered to be a spurious reaction.

The nature of the binding of lipid to protein and the relevance of such complexes to histochemistry and the composition of nuclei are discussed.

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