c\delta t_{\rm obs}/(1+z)$ by simple light-travel time arguments. Under the assumptions of isotropic radiation and Eddington-limited accretion, the implied minimum black hole masses of blazars are $\geq 8\times 10^{11}$ $M_\odot$ for PKS 1622-297 (Mattox et al. 1997b) from EGRET observations and $\geq 7.5\times 10^{8}$ $M_\odot$ for PKS 0528+134 from COMPTEL observations (Collmar et al. 1997).

Luminosity
==========

The $\gamma$-ray luminosity can be estimated by considering the relationship between the observed differential energy flux $S_0(E_0),$ where the subscript “$0$” denotes the observed or present value, and $Q_e$ the power emitted in $dE$, where $E=E_0(1+z)$ in the Friedman universe. $$Q_e[E_0]=4\pi S_0(E_0)(1+z)^{b-1}\Theta {D_L}^2(z,q_0)\eqno (1)$$ where $$D_L={c\over{H_0{q_0}^2}}\bigl[ 1-q_0+q_0z+(q_0-1)(2q_0z+1)^{1/2}\bigr]\equiv{{cz}\over{H_0}}g(z,q_0).\eqno (2)$$ $H_0$ is the Hubble parameter, $q_0$ is the deceleration parameter, $b$ is the spectral index, $z$ is the redshift, and $\Theta$ is the beaming factor. $H_0$ is chosen to be 70 and $q_0$ to be 0.5, although the results obtained here are not highly sensitive to these choices. The beaming factor is taken to be 1. The spectral index is obtained using the analysis described in §4. The luminosity as a function of the redshift is determined using equation (1) and is plotted in Figure 4 for the blazars detected by EGRET. The typical detection threshold for EGRET as a function of $z$, for relatively good conditions, is also shown in the figure. The actual threshold varies somewhat with exposure and region of the sky, and the average threshold is a little higher than the curve shown, but the shape is the same. The BL Lac objects are indicated in the figure with dark diamonds, and one sees clearly that they are predominantly closer and lower in luminosity.

Recently, Chiang & Mukherjee (1998) have calculated the evolution and luminosity function of the EGRET blazars, and have estimated the contribution of this source class to the diffuse extra