Objective: To assess whether human sera positive for antisperm antibodies have detectable levels of Sp17 autoantibodies and to determine the linear B cell epitopes to which these are raised for both native and recombinant Sp17. Design: Enzyme-linked immunoaborbent assays were performed against recombinant HSp17 on 15 serum samples from prevasovasostomy and postvasovasostomy patients. Positive sera then were used in mimotope analyses to determine HSp17 immunodominant linear B cell epitopes. These were compared with the linear B cell epitopes of recombinant HSp17. Setting: University research laboratory. Patient(s): Fifteen vasectomized or vasovasostomized men. Main Outcome Measure(s): Serum antibody reactivity to human Sp17. Result(s): Sera from vasectomized and vasovasostomized men exhibit Sp17 antibodies raised predominantly to two immunodominant linear B cell epitopes (amino acids 4 to 19 and amino acids 118 to 127), which differed from those of recombinant HSp17 (amino acids 52 to 79 and amino acids 124 to 136). Conclusion(s): The results show that Sp17 is an antigen to which vasectomized men raise autoantibodies. Two linear B cell epitopes predominate in native Sp17 and these differ from (but overlap with) those of the bacterially expressed recombinant protein.