Abstract Using the “linear plate” immunodiffusion technique, the authors have determined the serum IgG, IgM and IgA levels in normal populations. The incidence of storage of the samples at −20° for a prolonged time was investigated and also the influence of age on immunoglobulin levels in a normal population. The reference sera were obtained by mixing a great number of serum samples (four pools were obtained for each immuniglobulin) containing the protein at markedly different levels. The immunoglobulin concentration of each pool was determined by comparison with a calibration curve obtained with well weighted quantities of immunochemical pure human IgG or IgM or IgA powder. The frequency distribution of the immunoglobulin levels in the populations investigated was found to be of the log normal type. The mean levels (1275 mg/100 ml for IgG; 74 mg/100 ml for IgM and 208 mg/100 ml for IgA) fit very well with the data reported by most authors using other methods. The determination of the IgG level was not influenced by storage of the samples at −20°. A decrease of IgM and IgA was, however, observed when sera were kept frozen for a period of 3 months. No significant variation was found between the IgG and IgM levels of the different age classes. For the IgA's, however, the authors found a slight but significant increase between 20 and 30 years. Between the older age classes the increase is slower and between 40 and 60 there is no longer a significant change.