The toxin alpha-sarcin specifically cuts 28 S rRNA at a single position 393 nucleotides from its 3' end in isolated rat liver polysomes, provided the ribosomes are pretreated with EDTA or puromycin (Endo, Y. & Wool, I. G. (1982) J. Biol. Chem. 257, 9054-9060). In addition, alpha-sarcin behaves as a purine-specific RNase on deproteinized RNA, cleaving on the 3' side of purines in both single- and double-stranded RNA (Endo, Y., Huber, P. W., and Wool, I. G. (1983) J. Biol. Chem. 258, 2662-2667). Since alpha-sarcin does not readily enter tissue culture cells, we have injected it into Xenopus oocytes in order to determine whether the toxin cleaves after all purines or if it specifically makes a single cut in 28 S rRNA in intact cells. We report here that in oocytes alpha-sarcin specifically cuts 28 S rRNA 377 nucleotides from its 3' end, even when used at concentrations that would degrade deproteinized RNA. alpha-Sarcin does not behave as a general nuclease when injected into Xenopus oocytes nor does it operate by another means such as initiating proteolytic digestion of endogenous oocyte proteins. We demonstrate that injected alpha-sarcin causes a rapid decline in oocyte protein synthesis for soluble cytoplasmic proteins, similar in effect to injection of cycloheximide or puromycin.