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A derivation of the \gamma\gamma --> e^+ e^- optical depth for \gamma rays produced in a comoving spherical emitting region is presented. Employing a simplified expression for the \gamma\gamma absorption cross section, analytic expressions for the minimum Doppler factor implied by the requirement of gamma-ray transparency are derived for a broken power-law spectrum of target photons which are isotropically distributed in the comoving frame. Application to specific systems is illustrated

PHOTOABSORPTION OF GAMMA RAYS IN RELATIVISTIC JETS