Anti-arsonate hybridoma antibodies of A/J mice have been grouped into 3 VH families by amino acid sequence analysis. One of these, the 91A3 family, which includes all CRI-positive molecules and some CRI-negative molecules, has been serologically defined by the presence of a VH marker (91A3 marker). Although the 91A3 family of antibodies is present at low levels in preimmune A/J serum and represents a substantial component of the A/J response to p-azophenylarsonate, the 91A3 family has not been detected serologically in the anti-Ar response of any other inbred strain tested. Analysis of sera from AXC Igh recombinant mice maps the 91A3 marker to the left of Igh-Dex, consistent with the location of the VH genes controlling the arsonate CRI. These studies establish a serologic assay that detects all members of a family of anti-arsonate antibodies defined previously by primary sequence analysis. This assay makes it possible to monitor in its entirety the expression of the products of a single or closely related group of genes.