A laboratory study was conducted to determine the effects of the speed of lifting and box size on isokinetic strength and to compare isokinetic lifting strengths with static lifting strengths and psychophysically determined maximum acceptable weights. Nine male college students lifted three different boxes (250, 380 and 510 mm wide) from the floor to a bench height of 0.8 m using a free-style lifting technique at a rate of 0.2 lifts min-1. For each lifting task static strength was measured at the origin of lift. Isokinetic lifting strength was measured at 0.41, 0.51 and 0.6 ms-1 using a Biokinetic ergometer and attaching boxes to the load cell. Ratings of perceived exertion were recorded for the low back. There was a progressive decrease in mean and peak isokinetic lifting strengths both with an increase in lifting speed and with an increase in box width (p < 0.01). The lifting speed had a much greater effect (29% and 27%) than the box width (18% and 15%) on mean and peak isokinetic lifting strengths. However, high speed lifting was perceived subjectively to be less stressful (RPE = 10.7) than slow speed lifting (RPE = 12.7). Static strength and maximum acceptable weight had higher correlations with mean isokinetic strength (r = 0.65 and 0.82) than with peak isokinetic strength (r = 0.52 and 0.73). At 0.41 ms-1, mean isokinetic strength peak 6% greater than the mean static strength (p > or = 0.05). Extrapolation of mean isokinetic strength data showed that at 0.73 ms-1 the estimated mean isokinetic strengths were within 6% of maximum acceptable weights. It is concluded that isokinetic strength is highly dependent upon the speed of lifting. At a slow speed (0.41 ms-1), mean isokinetic strength is equal to mean static strength; and, at a high speed (0.73 ms-1), it appears to be equal to the maximum acceptable weight. It is recommended that both speed of lifting and box width should be controlled carefully to stimulate job-specific isokinetic lifting strength.