Analysis of the antigenic structure of the E. coli ribosomal protein S1 was undertaken using a set of 13 monoclonal antibodies (MAbs) directed against the isolated S1. The location of the epitopes was mapped using a series of large fragments and truncated forms of S1. Most of the epitopes were localized in the C-terminal half of the molecule, while only one antibody bound to the N-terminal region. Two MAbs were able to bind to more than one region of S1, suggesting the presence of repeated epitopes related to internal sequence homologies. Six distinct antigenic domains were identified by competitive binding assays. Competition between some antibodies suggested that the C-terminal region of S1 might be in spatial proximity with the N-terminal domain in the tertiary structure of the protein. The binding of a few MAbs induced conformational changes in the protein which resulted in the complete inhibition of antibody binding at non-adjacent sites. All the MAbs reacted with the isolated form of S1 or with the protein bound to the small ribosomal subunit. This indicated that the same epitopes were expressed in the two forms of the antigen and that they were accessible to antibody binding when S1 was part of the ribosomal subunit.