Aluminium in urine (AlU) and in plasma (AlP) was determined in seven subjects occupationally exposed to environmental concentrations of aluminium below or equal to the TWA (5 mg/m3). The AlU levels in these workers were markedly higher than those found in the control group. The levels of the indicator were definitely higher at the end of the shift than at the beginning of the same working day; also, the AlU levels were higher on Friday morning than on Monday morning. After an interruption in work of two weeks, the values of the indicator underwent a marked reduction and were then only slightly higher than those of the control group. Occupational exposure to fumes produced higher AlU levels than exposure to dusts, and in the subjects exposed to fumes the AlU levels were clearly influenced by the degree of exposure. The levels of aluminium in plasma in the exposed workers on the other hand, hardly differed from the levels found in the control group. These data appear to indicate that, whereas AlU allows daily and weekly exposure to be evaluated, AlP cannot be used as an indicator of occupational exposure, at least in the case of brief exposures to environmental concentrations below or equal to the TWA.