Chromatography, on diethylaminoethyl cellulose, of leg muscle extracts from older chick embryos yields three myosin-containing fractions, whereas material from muscle extracts of 11-day embryos cannot be eluted under the same conditions. A major fraction having the adenosine triphosphotase activity characteristic of myosin can be eluted when 11-day-old embryo muscle extracts are given prior treatment with ribonuclease.