The specific serum antibody responses of sheep exposed to 10 consecutive infections of L. cuprina have been analysed by enzyme-linked immuno-sorbent assay and immunoblotting using monoclonal antibodies specific for sheep immunoglobulin isotypes. Recognition of a number of larval excretory-secretory products by IgM antibodies appeared to be non-specific. IgG1 was the major antibody class stimulated by the infection protocol and marked increases in antibody to specific excretory-secretory antigens were observed. Three molecules of 35, 30 and 25 kDa were particularly recognized although the extent of recognition of these molecules varied considerably between individual sheep serum. A pooled serum composed of sera collected after five to seven infections significantly inhibited larval growth in in vitro cultures when compared to a sera pool consisting of sera collected both prior to infection and after infections 1 and 2. The degree of inhibition was greater when serum with high specific antibody titre was used.