In vitro antibody responses to a synthetic immunogen, consisting of both a B cell [V3 loop of gp120 from human immunodeficiency virus type 1 (HIV-1)] and a T-helper epitope (15 amino acids of tetanus toxoid) was studied. The in vitro activation was performed by primary and secondary in vitro immunizations, using lymphocytes obtained from uninfected, seronegative donors. Analysis of the in vitro immune response demonstrated an antigen-specific isotype switch, which was dependent on the presence of antigen-specific T-helper cells, CD40 ligation and antigen. Antibody libraries were constructed from cells derived directly from the naive donors, or from primary or secondary in vitro immunized B cells. Five libraries were displayed on filamentous phage and selected for anti-V3-specific Fab fragments, using a selection approach that linked recognition and phage replication. A panel of 19 recombinant antigen-specific Fab. representing different phases of the humoral in vitro immune response were sequenced, expressed and analysed using a biosensor. Recombinant Fab fragments derived from cultures on day 12 exhibited an increase in affinity of close to two orders of magnitude compared to those obtained from cells primary immunized for 7 days. This study provides the first evidence that an antigen-specific in vitro immune response can yield high-affinity immunoglobulinG antibodies.