The solid phase enzyme linked immunosorbent assay (ELISA) has been used to quantify anti-keyhole limpet haemocyanin (anti-KLH) antibody in the serum of KLH-immune C57Bl/6 mice. When spleen cells from immune mice were cultured overnight in ELISA microtitre wells to which KLH had been adsorbed it was found that easily quantifiable amounts of anti-KLH antibody were synthesized and were detectable. It was found further that spleen cells from KLH-primed mice, when cultured in vitro in the presence of KLH, transferred to KLH-labelled ELISA plates, and cultured overnight, also produced detectable levels of antibody. Levels of antibody were detectable only after 4 and 5 days of in vitro stimulation. A comparison was made between detectable numbers of plaque forming cells to sheep red blood cells (SRBC) in SRBC primed CBA mice and levels of antibody detected by the ELISA procedure. It was found that the sensitivities of the two tests were comparable. The applications of this technique to the study of in vitro antibody synthesis using soluble antigens are discussed.