Lung function was measured by spirometry in 66 workers in a sisal ropemaking factory, and in their matched controls. The major atmospheric contaminant was the lubricant (or a component part thereof) used to soften the fibre. The concentration of airborne matter was generally less than 1 mug m--3. There was no difference in lung function between the two groups before the start of the working shift, that is, the mixture of softening lubricant and sisal caused no long-term effects. Although there was no change in lung function over the working shift in the group making sisal rope, the control group did show a significant increase in lung function over the same period. This suggests that an effect attributable to the lubricant and sisal dust did exist. In previous studies little mention has been made of the softeners used in the processing of sisal fibre. These additives may exert a significant effect on ventilatory capacity and may act in conjunction with sisal dust.