Observations on the actin content of the rabbit myofibril.

1. On extraction of whole muscle by the procedure of Hasselbach & Schneider (1951), the amount of actin that passes into solution seems to account for little more than 10% of the protein content of the myofibrils. 2. Extraction of isolated myofibrils with suitable media that allow identification and estimation of dissolved proteins seems to give about the same yield of actin (10-13% of the total). 3. A comparatively large residue of myofibrillar components remains after extraction. The amount of actin present in the residue can be only hypothetical.