A sensitive immunoassay is described for the detection of idiotype- and isotype-specific antibody-secreting cells (ASC), based upon the well established principles of ELISA. Single cell suspensions containing ASC are incubated on solid phase to which specific antigen has been chemically conjugated. Antibody attaches to the latter within the immediate microenvironment of the ASC, producing localized zones of bound antibody which are subsequently developed as visual 'spots' in the ELISA. This assay system has sensitivity and specificity at least equivalent to haemolytic plaque assays.