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The relationship between the binding domains of elongation factor G (EF-G) and stringent factor (SF) on ri- bosomes was studied. The binding of highly purified, radio- actively labeled, protein factors to ribosomes was monitored with a columri system. The data show that binding of EF-G to ribosomes in the presence of fusidic acid and GDP or of the noncleavable analogue GDPCP prevents subsequent binding of SF to ribosomes. In addition, stabilization of the %ere are two classes of factor-dependent reactions for nu- cleoside triphosphates on the ribosome. One of these is the hydrolysis of GTP to form GDP and inorganic phosphate as, for example, in the reactions mediated by the elongation factors (Lucas-Lenard & Lipman, 1971). The other is a reaction of GTP with ATP to form ppGppI and pppGpp, the so-called magic spots MSI and MSII, which are produced by stringent factor (Haseltine et al., 1972; Pedersen et al., 1973; Sy & Lipmann, 1973). The functional relationship between the ribosomal domains responsible for these two kinds of nucleotide reactions is the subject of the present report. That the ribosomal domains associated with the hydrolytic reactions of GTP are functionally coupled to each other, if not overlapping, is suggested primarily by two lines of evidence. First, the elongation factors G (EF-G) and Tu (EF-Tu) cannot be bound simultaneously to the ribosome (Cabrer et al., 1972; Miller, 1972; Richter, 1972; Modolell