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The preparation, properties, and immunogenicity of the dialyzable factor from a ribosomal fraction of Salmonella typhimuriumare described. The ribosomal fraction was purified to eliminate O‐antigenic components, by affinity chromatography (Sepharose‐anti‐O antibody conjugates used as immunoadsorbent). The dialyzable factor was obtained in the concentrated dialysate of the purified ribosomal fraction which was alternately frozen in dry‐ice acetone and thawed in an 80 C water bath, for a total of five or six cycles. When this preparation was tested for its ability to protect mice against challenge with 1,000 LD50 of the homologous bacteria, it afforded 100% protection at a dose equivalent to 5.0 μg of RNA. The protection conferred by this factor was mainly cell mediated but immune serum enhanced this immunity despite the fact that no antibodies were detected in it. The protective activity of this factor was sensitive to RNase digestion but resistant to proteolytic enzymes. Ion exchange chromatography of this factor with DEAE‐Sephadex A‐25 (in 7 MUrea‐0.02 MTris‐HCl buffer, pH 7.5) resulted in a single A260peak which was found to be immunogenic. Chemical analysis of this peak after it was concentrated and desalted revealed that this immunogenic fraction was composed mainly of mixed nucleotides. The data indicate that protective immunity conferred by a ribosomal vaccine is associated with RNA but may not require the intact RNA molecule.