Based on an earlier described macromethod for the routine measurement of IgM and IgG in rat sera, a mechanized micro enzyme-linked immunosorbent assay (ELISA) was developed. The assay was performed in the wells of microtiter plate thus minimizing the quantities of reagents and antisera needed. Data on reproducibility of the assay and calculation of IgM and IgG levels are provided. For the functional assessment of the humoral immunity of the rat, ELISA is a powerful tool. In an earlier report, assays for the titration of thymus-independent IgM antibodies to E. coli LPS and the IgM and IgG response to the thymus-dependent antigen tetanus toxoid were described. More recently it was shown that the antigen ovalbumin elicits a thymus-dependent IgM, IgG and IgE response which could be readily measured with the enzyme immunoassay, as well as a delayed-type hypersensitivity reaction. As the optimum ovalbumin concentration for both types of reactions was the same, it is concluded that the ovalbumin model offers the advantage that both humoral and cellular immunity can be studies simultaneously in the same animal.