Fucose-labelled proteins were examined for the release of low-Mr O-linked fucose substituents after mild alkaline-borohydride treatment. A component tentatively identified as glucosylfucitol (DS) and an apparently higher-Mr component (TS), which also contained fucitol, were observed to be released over a broad molecular-size range of proteins. Approx. 90% of the DS-releasing proteins were in the particulate fraction, whereas only approx. 66% of the TS-releasing proteins were in that fraction. In addition to cell-associated proteins, a substantial proportion of DS-containing proteins were shed into the medium. For example, after 96 h of labelling there was 6-fold more of these components in the growth medium than were cell-associated. Moreover, the incorporation of labelled fucose into both the DS and TS appeared to be cell-population-density-dependent. Despite the apparent wide distribution of these novel fucose substituents in cellular proteins, it seems reasonable to suggest that they have not been routinely observed largely because each represents less than 0.5% of the fucose bound to protein.