BIOCHEMICAL SOCIETY TRANSACTIONS chloroquine (see Ohkuma & Poole, 1981; Danpure et al., 1983), but rather its effect on the uptake of cystine. These and other data (see Danpure et al., 1984~1, b) enable two routes of translocation of radioactivity from exogenous cystine t o intralysosomal cystine in cystinotic fibroblasts to be postulated. Route 'A' is fast, is inhibited by chloroquine (type 'A' effect) and increases with increasing pH. This route almost certainly starts with the plasma membrane transport system described by Bannai & Kitamura (1981). Route 'B' is slow, is stimulated by chloroquine (type 'B' effect) and may be nearly independent of extracellular pH. This route may start with pinocytosis. As the pH is increased the proportion of cysteinyl moieties flowing through route 'A' is increased, so that the inhibitory type 'A' effect of chloroquine is also increased. Similarly as the proportion flowing through route 'B' is decreased, so the stimulatory type 'B' effect of chloroquine is diminished, or even nonexistent at the highest pHs. At the low pHs the plasma-membrane transport system of Bannai & Kitamura (1981) (route 'A') will be considerably inhbited, leading to a lessening of the chloroquine type 'A' effect. The increased relative importance of route 'B' (pinocytosis?) is manifested by the maximal chloroquine type 'B' effect at the lowest pHs.