Structural biology is a branch of life science concerned with the study of the structure of biological macromolecules like proteins. The structure of a protein gives much more insight in its functions than that of its amino acid sequence. Protein structure comparison is important for understanding the evolutionary relationships among proteins, predicting protein functions, and predicting protein structures from the chemical composition. In this paper we propose a new approach for structural block retrieval based on the Generalized Hough Transform (GHT). A first technique uses as primitives the single Secondary Structure (SS), an alternative adopts co-occurrence of SSs couple, the third approach uses SSs triplets, and finally the primitive can be an entire block. In this paper we describe some experiments for the retrieval of elementary structural blocks consisting of four- and five-SSs.