SUMMARY1. The activity of the enzyme catechol‐O‐methyltransferase (COMT) was determined in the erythrocytes of normal and autistic children.2. There was no difference in enzyme activity between the two groups, although in both the normal and autistic females the erythrocyte COMT activity showed considerable within‐group variation.3. When the erythrocyte homogenates were divided into particulate and soluble fractions, some differences were observed. The COMT activity in the soluble fraction was less in erythrocytes from autistic males than in those from control males, whereas that in autistic females was higher than in those from control fe