1. One aim of this paper is to show an alternative approach for the determination of antagonist affinity estimates, KB and pA2, by construction and evaluation of antagonist dose-response curves (DRCs), using the curve-fitting programme, ALLFIT. 2. Parallel antagonist DRCs were derived by vertical analysis of families of conventional agonist DRCs in the presence and absence of an antagonist at a certain agonist concentration above its ED50. The latter represents a chosen, i.e. fixed dose-ratio (DR). The antagonist concentration that reduces an agonist effect to its Emax/2 was termed Bx. It corresponds to B, the fixed antagonist concentration, tested to obtain DR-1, conventionally. 3. The dissociation constant was calculated as KB = Bx/DR-1, analogous to the conventional approach (KB = B/DR-1). Likewise, pA2-values were estimated by plotting log Bx, obtained by the alternative approach, vs log (DR-1) in an 'alternative Schild plot'. 4. Experimental agonist DRCs from our laboratory and from the literature were analysed and KB- and pA2-values obtained by the alternative approach were compared with those obtained by the conventional method. The results showed a very good agreement (correlation) between the pA2-values obtained by either method (slope = 1.02, r = 0.99, n = 9), in agreement with theoretical DRCs. 5. Besides estimation of KB and pA2, antagonist DRCs were also evaluated qualitatively. The most important finding was that allosteric antagonists or competitive antagonists with an allosteric component, such as gallamine, showed a significant reduction in the maximum of the antagonist DRCs (Imax). The evaluation of antagonist DRCs appears to be a sensitive procedure to detect allosteric interactions.6. This alternative approach can supplement or replace the conventional approach for the evaluation of antagonists on a quantitative and qualitative basis. The alternative approach appears of special advantage where the supply and/or the solubility of the agonist is limited, resulting in incomplete agonist DRCs.7. For rapid screening of potential antagonists, a single antagonist DRC at the maximum effective agonist concentration may be constructed to calculate KB reliably.