The shock model of gamma-ray bursts (GRBs) has two free parameters: B and e. It has been shown that B should range between few x 10(-3) and few x 10(-4). However, how to calculate the value of e has remained an outstanding theoretical problem for over a decade. Here, we demonstrate that the Weibel theory inevitably predicts that epsilone approximately radical epsilonB. The GRB afterglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. This e-B relation can potentially be used to reduce the number of free parameters in afterglow models.