In a complement-dependent assay, using protein A-coupled erythrocytes as target cells and anti-albumin immunoglobulin (Ig) as a developing serum, it was possible to detect single albumin-secreting hepatocytes as plaque-forming cells. Plasmacytoma cells were detected using anti-Ig antibodies and did not form plaques with anti-albumin Ig. This method can potentially be used for detecting any cell-secreting products in sufficient amounts.