We have developed a solid-phase assay for the quantification of terminal deoxynucleotidyl transferase (TdT) enzymatic activity in crude cellular extracts. Affinity-purified, polyclonal anti-TdT antibodies are bound to the wells of a microtiter plate, and TdT in extracts is then bound to the immobilized antibodies. The enzymatic activity of the antibody-bound TdT is measured directly in the wells of the microtiter plate. This method yields highly reproducible values, even with samples of low activity. Because it is also technically very simple, it is ideal for determining enzymatic activity for large numbers of clones with limited numbers of cells.