SPR of fetal calf muscle nicotinic receptor binding to HIV-1 gp120. Triton-extracted membranes from fetal calf muscle (FAchR) were diluted in 10 mmol/L acetate buffer, pH 6.5, 150 mmol/L NaCl to a concentration of 0.5 nmol of ¹²⁵I-α–bgt bound/liter and then injected at a flow rate of 5 μL/minute over a matrix where HIV-1 gp120 had been covalently immobilized. The same sample was injected in the presence of increasing concentrations of nicotine. Regeneration of the matrix was obtained by injection of 10 mmol/L NaOH at the end of each cycle.