The evaluation of contact reactions in previously sensitized mice is assessed conventionally by measurement of increases in ear thickness following challenge. In an attempt to develop a serological method for the investigation of contact sensitization in mice, we have examined whether analysis of changes in the concentration of acute-phase proteins in response to challenge provides a reliable alternative means of evaluating elicitation reactions. Measurement of either the relative serum haptoglobin concentration, using radial immunodiffusion, or the absolute concentration of serum amyloid A, by an enzyme-linked immunosorbent assay, has been found to correlate well with induced increases in ear thickness following challenge. Changes in the concentration of acute-phase proteins proved to be of sufficient sensitivity to reflect the specificity of contact sensitization and its inhibition by antigenic competition.