Steroids form a structurally closely related group. As a result, antibodies produced for use in immunoassays regularly show unwanted cross‐reactivities. These may be reduced by altering hapten‐protein coupling procedures, thereby reducing the exposure of the determinants giving rise to the undesirable cross‐reaction. However, these procedures can prove to be complex, expensive and not totally predictable in outcome. Exploitation of the clonal selection theory is an attractive alternative approach. The host is primed with the interfering cross‐reactant coupled to a non‐immunogenic amino acid copolymer to inactivate the B‐lymphocyte clones specific for this steroid, producing a specific immunotolerance. Then, 3 days later, the host is immunized with the steroid against which an antibody is required. The clones producing antibody to this immunogen are unaffected and the cross‐reactivity is significantly reduced or deleted. The technique has been applied to the reduction of endogenous sex steroid cross‐reactivity from antibodies prepared against synthetic and semi‐synthetic androgens (17α‐methyltestosterone, 19‐nor‐ß‐testosterone) and the progestogen medroxyprogesterone. Antibodies prepared against the synthetic oestrogen zeranol using this technique have significantly reduced its undesirable cross‐reactivity with the fungal metabolite 7α‐zearalenol. Highly specific antisera have been generated in all cases, the only adverse effect being a reduction in the titres achieved in comparison with rabbits receiving the conventional immunizing regime.