In a primary infection, the numbers and proportions of germinal centre B cells (CD19+GL7+) increased with kinetics similar to other activated B cell subsets with a maximum of 8·9% (day 25 ± 1·8%) of all CD19+ B cells and returned to the level of age-matched, uninfected mice by 90 days (data not shown). This evidence concerns the gene CD19 and infection.