A study on combined manual materials-handling tasks performed on floors under three friction levels was conducted. Eight male subjects participated in the study. The maximum acceptable weight of handling, including lifting, carrying for 3m, lowering, and walking 3m back at twice per minute was determined. The subject then performed the same tasks for 10 min. Heart rate, Vo2, energy efficiency, perceived sense of slip, and rating of perceived exertion for whole body strain were measured. The results showed that the effects of friction level on the maximum acceptable weights of handling, perceived sense of slip, Vo2, and energy efficiency were statistically significant (p<or=0.0006). As the friction level increased from low to high, the maximum acceptable weights of handling increased from 8.15 to 9.34 kg. The energy efficiency on the low friction condition (12.58 kg/L/min) was significantly lower than those of the medium (15.73 kg/L/min) and high (15.38 kg/L/min) friction conditions. The perceived sense of slip was the highest (5.44) on the low-friction condition, followed by the medium-friction condition (3.58), and last the high-friction condition (1.84). The implication of this study was that friction level should be regarded as one of the major environmental factors in designing MMH tasks as it affected both physiological and psychophysical responses of the subjects. Low-friction footwear-floor interface should be avoided as it resulted in not only high scores of perceived sense of slip but also in low-energy efficiency utilized in the body.