A random fragment expression library was used to identify and map a new human epitope on the vpu protein of the human immunodeficiency virus type 1 (HIV-1). The epitope was mapped to the central part of the protein within amino acids (aa) 37-50 comprising the sequence N-KIDRLIDRLI-ERAE-C. A alpha-galactosidase-vpu fusion protein representing aa 37-68 of vpu was used to screen 356 human serum samples from HIV-1-infected persons for antibodies to the novel epitope. A total of 125 (35.1%) of the samples reacted with this region of vpu. Antibodies against this region were significantly more prevalent among samples from individuals with CD4 cell counts < 400 cells/microliters than individuals with CD4 cell counts > or = 400 cells/microliters (37.6 vs. 17.6%; p < 0.0146, Fisher's exact test). Thus, the presence of antibodies against this epitope of vpu appears to be associated with a progressed state of disease.