A statistical analysis is presented of the results obtained from the measure of the serum concentration of IgG, A, M in 773 blood donors. In each of the two groups identified by sex, four subgroups were established according to age in ten years intervals from 20 to 59. On the basis of the results of the analysis, different fiducial intervals of normal values (calculated as mean +/- 2 X standard deviation) have been proposed. 1) A unique interval for IgG: 87-234 U.I. 2) Two intervals for IgA: 54-236 U.I. for subjects under 40 years and 64-270 U.I. for subjects over 40 years. 3) Four intervals for IgM: one for males under 40 years: 65-364 U.I., one for males over 40 years: 57-314 U.I., one for females under 50 years: 47-363 U.I., and one for females over 50 years: 53-434 U.I.