Immunodiffusion of antigen from conic and cylindrical wells of various sizes in thick antiserum gel plate was studied. The shape of precipitate was affected by the shape and size of the well and by the antigen weight. In the test of anti-rabbit serum agar plate, albumin concentration ranged from 2 to 25 mg/ml, and the maximal albumin weight was 15.8 times the minimal one. In this case, regression line and standard deviation (S.D.) were calculated from antigen weight (x) and standardized data (y1) of precipitate volume, and from antigen weight and standardized data (y2) of precipitation ring area, and the following results were obtained: y1=0.006x-1.611, S.D.=0.092 for the former and y2=0.005x-1.546, S.D.=0.295, for the latter. In the test of agar plates containing anti-carbonic anhydrase B (CA B), the maximal concentration and weight of CA B was 16.7 times and 18.3 times the minimal one, respectively. In this case, regression line and standard deviation obtained from the CA B weight (x) and the standardized data (y3) of the precipitate volume, and from the weight (x) and the standardized data (y4) of the precipitation ring area were as follows: y3=0.146x-1.754, S.D.=0.208, and y4=0.104x-1.251, S.D.=0.717, respectively. There was a clearer linearity in the relationship between the precipitate volume and the antigen weight than in that between the ring area and the antigen weight. In the gel medium, antigen concentration gave no influences on antigen quantitation at the end point of antigen diffusion in a wide range.