The trypanocidal drug suramin, an impermeant polyanion, has been shown to be a powerful inhibitor of the calcium uptake and calcium-stimulated ATPase activity of sarcoplasmic reticulum (Fortes et al., 1974). In view of this finding, an attempt was made to investigate the effect of suramin on Ca<sup>++</sup> transport in resealed red cells and on Ca<sup>++</sup>-activated ATPase in red blood cell membrane fragments (RBCMF). The results obtained are summarized as follows. 1. Ca<sup>++</sup> outflux from the resealed RBC was inhibited by suramin and the inhibitory action of suramin is proportional to the concentration of drug added inside the RBC preparation. When suramin is added both inside and outside the RBC preparation simultaneously, the magnitude of the inhibitory effect was more pronounced, suggesting that suramin inhibits both active Ca<sup>++</sup>-<sup>45</sup>Ca exchange diffusion across the RBC membrane. 2. Suramin inhibits the Ca<sup>++</sup>-activated ATPase of the RBCMF and the effect of inhibition by the drug was also concentration dependent. From the above results, it may be concluded that suramin inhibits Ca<sup>++</sup> transport across RBC membrane by inhibiting Ca<sup>++</sup>-activated ATPase activity which has been known to be linked with active Ca<sup>++</sup> transport.