1. A ribosome-independent GTPase activity has been isolated from the high-speed supernatant fraction of Artemia salina embryos, and some of its properties have been studied. This activity is inhibited by fusidic acid, an antibiotic generally thought to inhibit only EF-2 in eukaryotes. However, several lines of evidence indicate that the GTPase activity, described here, is distinct from EF-2. The results suggest, therefore, that the inhibitory effect of fusidic acid in eukaryotic systems is not restricted to EF-2 (and ribosome)-dependent functions only. 2. The results of other experiments have revealed that, despite its ability to inhibit the GTPase activity mentioned above, fusidic acid is not a non-specific inhibitor of all ribosome-independent GTPase and ATPase activities present in eukaryotic cells.