Skin fibroblasts from healthy individuals and a mannosidosis patient were cultured in the presence of [2-3H] mannose and the cell homogenates were fractionated by trichloroacetic acid precipitation into a precipitable and a non-precipitable portion. In uptake as well as in chase experiments the precipitable fractions show no significant difference in their content of radioactivity, while an increased level of radioactivity is found in the non-precipitable fraction of mannosidosis cells. This higher radioactivity content is due to a higher mannose content and is caused by a slower degradation of this fraction. The differences between the metabolisms of the two cell lines can be expressed by the ratio of radioactivity in the non-precipitable and the precipitable fractions. This value is about three times higher for mannosidosis than for control cells. Pig kidney alpha-mannosidase is taken up by both cell lines and is able to correct the impaired degradation of the non-precipitable material in mannosidosis cells, as shown by the normalization of the above defined ratio of radioactivity for this cell type.