TABLE 1.

Binding of PA and LF to RAW264.7 cells^a

Temp (°C)	ng of toxin/mg of cell protein		PA/LF ratio
	PA	LF
4	2.25 ± 0.35	2.22 ± 0.19	1:0.98
37	9.15 ± 0.32	8.32 ± 0.27	1:0.91

^aRAW264.7 cells cultured in 12-well plates were incubated with 0.1 μg of ¹²⁵I-PA per ml alone or in combination with 0.5 μg of ¹²⁵I-LF per ml at 4°C for 12 h or at 37°C for 2 h. Cells were washed four times and solubilized in 0.1 M NaOH. The amount of LF bound to the cells was determined by subtracting the radioactivity in wells containing PA alone. Each value is the average of three individual wells and representative of a typical experiment performed three times. Nonspecific binding was determined by incubating the cells with nonradioactive PA or LF at 100-fold excess over labeled protein. The specific counts per minute per well and percentages of nonspecific radioactivity were as follows: 4°C, PA, 1,545 cpm (10.6%), and LF, 1,797 cpm (8.9%); 37°C, PA, 6,381 cpm (9.0%), and LF, 4,515 cpm (3.47%).