Data on a large, randomly selected population of apparently health subjects, aged 10 to 73 years, of both sexes were used to derive estimates of the reference intervals for serum levels of IgA, IgG, IgM, and IgD. The main effects of age and sex on the mean values were studied by the use of a two-way ANOVA model. Except for IgG that showed no sex dependencies, statistically significant sex and age-related differences in mean levels were found for each of the immunoglobulins studied. The mean concentrations of both IgA and IgD were higher in males than in females; an inverse relationship was identified for IgM. Serum levels of both IgG and IgA rose gradually with advancing age in either sex; by linear-regression analyses these age dependencies were found to be statistically significant for IgG in males and IgA in both sexes. The age-related changes in either IgM or IgD serum levels were more complex and fitteda quintic function better than a linear one. These results indicate that any interpretation of immunoglobulin values without appropriate consideration of the subject's age and sex is likely to be misleading.