Four biological properties, cytotoxicity to cell cultures, neurotoxicity to mice, capacity to increase xanthine oxidase activity of infected mouse lung, and neuraminidase activity of several early Asian isolates were compared to PR8 and FM1 influenza viruses. Among those properties, only the neuraminidase activity of the Asian strains to urine mucoprotein showed a significant difference. Asian isolates split off about 10-40% more N-acetylneuraminic acid, as determined by the resorcinol reaction, than PR8 and FM1. There were no differences in any of the properties between strains from mild and fatal human infection.