An acid-labile interferon-alpha (IFN-alpha) exists in IFN produced in human peripheral leukocytes with Sendai virus. This activity was found in fractions of molecular weights about 100 000 (100K), 58K and 44K. The IFN activity in the 100K fraction was neutralized with anti-HuIFN-alpha serum alone, but activity in the 58K and 44K fractions was neutralized only by a mixture of anti-HuIFN-alpha and anti-HuIFN-beta sera.