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EXPERIMENTS have been carried out to test the predictions of a new theoretical approach to the Oudin technique (Spiers and Augustin, 1957). The graphical method employed for this purpose is described in detail. The antigen (Ag)-antibody (Ab) systems used were crystalline Bovine serum albumin (BSa) and crystalline egg albumin (Ea), with the corresponding rabbit antiserum globulin concentrates. The experimental results fitted the theoretical curves best when both Ag and Ab were incorporated in gel. With Ag in solution the same theoretical curves fitted as for Ag in gel (except for the highest Ag/Ab ratios), but there was a slightly greater scatter of experimental points. The results supported throughout the assumption that the Ag diffused towards the interface and that the Ag concentration was not maintained homogeneous, as claimed by Becker et al. (1951). Conditions for the formation of a stationary line at the gel-gel or liquid-gel interface agreed with those predicted by the new theory rather than with those predicted by Becker et al. By means of the new method it is also possible to obtain estimates of Ag-Ab combining ratios (in the gel) together with a rough estimate of the ratios of the diffusion coefficients of Ag and Ab, neither of which had previously been possible. Diffusion coefficients determined by this new graphical method were about 15 per cent below the accepted values, and the combining ratios R had values in excess of equivalence ratios. Diffusion coefficients calculated from our experimental results according to the theory of Becker et al. (1951) were below those found by the new method. The reasons for this, and results obtained by previous workers, are critically discussed. The effect of variations of temperature and of absolute concentration of the reactants are also examined. [ABSTRACT FROM AUTHOR]