Cell-enzyme-linked immunosorbent assay (cell-ELISA) is a technique for the rapid, convenient, and quantitative detection of molecules expressed on the cell surface. Here we present an evaluation of beta-galactosidase as an antibody-tag for cell-ELISA. In contrast to substrates for horseradish peroxidase (HRP) and alkaline phosphatase, murine splenocytes do not hydrolyze the beta-galactosidase substrate chlorophenolred-beta-D-galactopyranoside (CPRG). beta-Galactosidase-antibody conjugates show much lower background binding to murine T cells than conjugates with HRP or alkaline phosphatase. We describe step-by-step procedures for direct and indirect beta-galactosidase based cell-ELISA to quantitate the expression of molecules on the surface of unfixed, live cells. Variations of the basic protocol are suitable for adherent and non-adherent cells, large scale screening for expression of cell surface molecules, and the screening of hybridomas for production of antibodies to cell surface epitopes. Since relatively few beta-galactosidase conjugated antibodies are commercially available, we describe an efficient method to couple beta-galactosidase to antibodies using a novel water soluble heterobifunctional crosslinker, sulfosuccinimidyl 4-[N-maleimidomethyl]-cyclohexane-1-carboxylate (sulfo-SMCC). We demonstrate the utility of this method by conjugating F(ab')(2) fragments of an anti-B7-2 antibody, and using this conjugate to assay B7-2 on Fc-receptor bearing cells.