The single-cell ELISPOT assay was used to determine the frequency and isotype commitment of virus-specific antibody-forming cells (AFC) at different anatomical locations following intranasal Sendai virus infection of C57BL/6 and 129/Sv mice. AFC responses in the mediastinal and cervical lymph nodes showed sharp increases and declines, first of IgM AFC, peaking about 7 days after infection, and then of IgG and IgA AFC, peaking about 10 days after infection. A wave of IgM AFC preceding the other isotypes was less evident in the spleen, where peak frequencies of AFC occurred 14 days after infection. Virus-specific AFC appeared in the bone marrow with a unique kinetic pattern, increasing in frequency gradually over the first 3 weeks after infection to a plateau that remained constant. Circulating IgM and IgG achieved significant titers approximately a week after infection; IgM titers were transient, but IgG levels increased sharply and remained high, reflecting the longevity of the bone marrow AFC response. Strain differences in isotype bias were noted, particularly preferential switching to the gamma 2a gene in 129/Sv mice. The B-cell response to acute respiratory viral infection thus exhibits features that are distinct from the primary response to nonreplicating antigens.