An influenza virus was isolated in May, 1991 from a child with acute respiratory disease. The identification results with serologic method have revealed that the isolate was A/PR/8/34 (H1N1)-like virus. The analyses of nucleotide sequences in HA1 domain showed that there were 12 different positions on nucleotide sequences between the isolate and the A/PR/8/34 (H1N1) virus. These differences induced substitutions of 4 amino acids in HA1 protein molecule of isolate as compared with that of A/PR/8/34 (H1N1) virus. The oligonucleotide analyses of viral genome revealed that there were 11 different points between isolate and A/PR/8/34 (H1N1) virus, e. g. 2 deleted and 9 inserted in genome of isolate as compared with A/PR/8/34 (H1N1) strain. Therefore, the isolate possibly derived from A/PR/8/34 (H1N1) virus by laboratory contamination could be ruled out. However, where did A/Guangdong/6/91 (H1N1) virus come from? This question will be settled by further study.