Affinity partitioning in dextran-polyethylene glycol-water biphasic systems has demonstrated that myosin has hydrophobic surface properties. In 0.5 M KCl at pH 7.5 myosin is transferred at increasing amounts to the polyethylene glycol-rich upper phase when an increasing proportion of that polymer in the system is replaced by its ester with lauric, myristic or palmitic acid. This shows that on its surface myosin has binding sites with affinity for long chain fatty acyl groups. Partition studies on the ionic strength range of 0.2-0.6 M KCl at pH 7.5 at 4 degrees C and 20 degrees C, respectively, in systems containing polyethylene glycol-palmitate showed that the affinity of myosin for the palmitate group becomes greater with (1) an increase in ionic strength, and (2) an increase in temperature at constant ionic strength. The affinity of myosin for the palmitate group also increases with a decrease in the pH in the range of 5.6-8.5. The increase in the affinity of myosin for the palmitate group parallels the increase in the tendency of myosin to self-interact and yield filaments.