It is shown that the empirical expression delta M/ delta H=X(0)-( beta /2)log(1+H²/K²) provides a very close fit to experimental spin-glass magnetic isotherms both at and above the freezing temperature T_g. Above T_g the parameter K is shown to vary in a very similar manner to X_nl(T) as reported by previous researchers. At T_g, however, K is found to be finite. It is argued that this result demonstrates that there are no divergent quantities in the spin-glass magnetic isotherm.