1. Sialic acid has been found to interfere with three colorimetric reactions used for the estimation of DNA: a modified diphenylamine reaction at 100 degrees (Dische, 1930), the nitrophenylhydrazine method (Webb & Levy, 1955) and the diphenylamine reaction at 30 degrees (Burton, 1956). 2. Evidence is presented that sialic acid is present in hydrolysates obtained from gastric wash-out material. 3. A mathematical method for correcting for interference from sialic acid in the diphenylamine reaction at 30 degrees is described. 4. The diphenylamine reaction has been modified to make it suitable for the estimation of DNA in the presence of sialic acid. The modifications are to increase the concentration of diphenylamine to 2% and to perform the reaction at 6-13 degrees for 48hr. These modifications increase the sensitivity 25% above Burton's (1956) modification of the diphenylamine reaction. 5. The precipitation, extraction and recovery of DNA from gastric wash-out material have been investigated.