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Direct microcalorimetric measurements allow determination of both the AH and association constant of biological complexes if Ka value does not exceed 106 M "1. For higher/Ca values, AH can obviously be determined; this paper describes an original microealorimetric method that permits determination of such high association constants. This method is based on the analysis of the competitive effect between two ligands having the same binding site in their receptor. As an example, the affini~ constant for thymidylate synthase of a novel antifolate, CB 3717, was found to be 1.4.10 M -1 using methotrexate polyglutamate MTX-G2 (Ka = 2.3 "105 M "I) as competitor.