The complete amino acid sequence of a guanyl-specific RNAse from Streptomyces aureofaciens has been established using a rapid method of primary structure analysis which eliminates the peptide fractionation. The automated Edman degradation of the carboxymethylated RNAse Sa and of non-fractionated peptide mixtures produced by tryptic and staphylococcal protease digests of the modified protein were used. The RNAse contains 96 amino acid residues, Mr 10,566. The secondary structures of RNAse Sa and microbial RNAses have been calculated using a modified Chou--Fasman procedure. A comparison of the primary and secondary structures of the RNAses revealed different degrees of sequence homology and a similar distribution of predicted structural regions (alpha-helices, beta-structure and beta-turn). The predicted secondary structure patterns are discussed in the light of the RNAse X-ray analysis date.