FACS analysis of anti-Id–IL-2 fusion proteins. A control MoAb (A and F), S5A8 (B and G), chS5A8 (C and H), chS5A8–IL-2 (D and I), or scFvS5A8–IL-2 (E and J) was reacted with 38C13 cells (A through E) or V1-1, an Id-negative variant of 38C13 (F through J). The bound anti-Id Abs were detected with second-step FITC-conjugated Abs or with second-step biotin-labeled Abs followed by FITC-labeled avidin, as detailed in the text. Dashed lines represent fluorescence from cells without addition of the tested anti-Id Abs.