The surface active properties of myosin and its proteolytic fragments, light meromyosin (LMM), heavy meromyosin (HMM), subfragment-1 (S-1) and myosin rod, at initial bulk phase concentrations in the range of 10(-4)% to 10(-2)% w/v were determined by the drop volume method. Overall, S-1 was the most effective surface tension depressor, whereas the tail portions of myosin, i.e. LMM and myosin rod were less surface active than the parent myosin molecule. The surface pressures attained after 40 min, at an initial bulk phase concentration of 10(-2)% (w/v), were 22·00, 21·77, 21·02, 16·77 and 16·77 mNm(-1) for S-1, HMM, myosin, LMM and myosin rod, respectively. Furthermore, S-1 effected the most rapid change in surface pressure during the initial 5 min period.