Isotype switch variants, which arise in monoclonal antibody-producing cell lines, can be detected and selected on the basis of sensitive isotype-specific assays. In this study we used a series of enzyme-linked immunosorbent assays specific for murine IgG1, IgG2b, IgG2a, IgE or IgA, which permitted the detection of low frequency switch variants of hybridoma cell lines, irrespective of the specificity of the secreted antibody. In these assays two rat monoclonal antibodies were combined: one specific for the particular heavy-chain isotype, the other for the light-chain isotype, which was identical in all variants. The value of rat monoclonal antibodies for the detection of isotype switch variants is illustrated by the isolation of a series of variant antibodies specific for the CD3 complex present on human T lymphocytes.