alpha-L-Iduronidase synthesis and maturation were analysed in fibroblasts from normal controls and from alpha-L-iduronidase-deficient mucopolysaccharidosis-type-I (MPS-I) patients. Fibroblasts were radiolabelled with [3H]leucine and alpha-L-iduronidase was isolated from cell lysates or culture medium by monoclonal-antibody affinity chromatography. Pulse-chase labelling of normal control fibroblasts showed that alpha-L-iduronidase was synthesized as an 81 kDa precursor and processed within 24 h via intermediates of 76 kDa and 70 kDa to a 69 kDa species. The incorporation of radiolabel into alpha-L-iduronidase in fibroblasts from three of four MPS-I patients was at levels that were either very low or undetectable. Fibroblasts from one MPS-I patient, however, exhibited levels of incorporation of radiolabelled amino acid into alpha-L-iduronidase similar to those shown by normal control fibroblasts, despite having undetectable alpha-L-iduronidase enzyme activity. The maturation of alpha-L-iduronidase in fibroblasts from this patient was delayed compared with normal controls and showed accumulation of the 76 kDa intermediate, as well as the major 69 kDa, form of the enzyme.