A novel inhibition procedure, called two-dimensional absorption-inhibition, is described. The theory underlying this technique is developed based on a review of and comparison with existing inhibition methods. Two-dimensional inhibition takes advantage of the best features of inhibition-titration and titration-inhibition, and is shown to be more sensitive than either of them. Results obtained using all the inhibition methods on secretor saliva, semen, urine, urine stain, and perspiration stain specimens show that the new technique is especially powerful in correctly determining the ABH antigens in secretor body fluids having lower concentrations of soluble blood group antigens. A two-stage version of the two-dimensional procedure that makes it a practical casework method is described as well.