In 588 sera of blood donors the Western Blot test (WB) was done for confirming the presence of anti-HIV-1 antibodies. Negative results were obtained in 247 sera and positive in 90. In 251 cases the result was doubtful (42.7%). These results were obtained in 153 cases, however only 104 subjects came for control tests and had from 1 to 5 tests. Eight subjects were returned to the register of blood donors after obtaining of two negative WB results. In 5 cases in control test the serum reacted with the protein of HIV-1, and they were regarded as being carriers. In the remaining 91 subjects doubtful results persisted for from 3 to 12 months. The reactivity of the doubtful sera changed. Most of them reacted with the proteins opceded by one of three genes, they were mostly core proteins, among them p24 protein. The remaining sera reacted with proteins encoded by genes of the envelope and core. The possible causes of these doubtful results are discussed.