The NOS activity of six different B-CLL cell samples was determined by measuring, in duplicate, the conversion of¹⁴C-L-arginine into ¹⁴C-L-citrulline, either by cell lysates for two of them (A) or by intact cells for the other four cases (B). B-CLL cells were incubated overnight in medium alone or containing 10 μg/mL of anti-CD23 MoAb or control IgG₁. Cell homogenates (A) were prepared at the end of the incubation time and their NOS enzymatic activity was tested over a 30-minute period. Alternatively, radiolabeled arginine was added at the beginning of the culture and the formation of radiolabeled citrulline in cell-free supernatants (B) was estimated at the end of the culture. Results (mean ± SD of duplicate samples) are expressed either in pmol/min/mg or pmol/10⁶ cells.