Using human monospecific antisera, several parameters have been optimized for the micro-ELISA "sandwich" technique used in the quantitative measurement of total serum IgG, IgM and IgA levels in rhesus monkeys. Representation of the optical density as a four-parameter logistic function provided excellent fits of the data over a wide choice of dilutions of human antisera used for coating the ELISA plates and for the peroxidase-conjugated antisera used in the system. The micro-ELISA "sandwich" technique was shown to be specific, reliable, sensitive, and economical for use in the routine measurement of total serum IgG, IgM and IgA levels in the rhesus monkey.