Covalently linked peroxidase-anti-human IgG conjugates were prepared by either glutaraldehyde or NaIO4 coupling techniques. Sodium dodecyl sulfate polyacrylamide gel electrophoresis shows that the glutaraldehyde coupled conjugate is composed of generally lower molecular weight components than the NaIO4 coupled product. The NaIO4 conjugate, when used to quantitate human immunoglobulin (Ig) in enzyme-linked immunoassays, appears to be highly sensitive in that small amounts of Ig elicited relatively high reactivities. The quantitative range of this type of conjugates, where reactivities are linearly proportional to the amount of human Ig present, is, however, extremely narrow (0.01-0.10 micrograms/ml of human IgG). Conversely, the glutaraldehyde coupled type conjugate is capable of sustaining a much wider range of linearity (0.01-0.6 micrograms/ml), but with a more gradual rise of reactivity which corresponds well to the amount of human Ig present. Conjugates prepared with glutaraldehyde are thus more useful in quantitative assays where wide quantitative ranges are desirable. NaIO4 conjugates on the other hand, are more suited to qualitative assays where sensitivity is more important.