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ABSTRACT We use multiwavelength data collected during the high state of BL Lacertae in July 1997 to study the high energy radiation processes that are likely to be responsible for the radiative output from relativistic jet in this object. We show that the broad-band spectrum measured in July 1997 can be explained in terms of the model where the GeV γ-rays are produced by Comptonization of broad emission line photons, while X rays result from Comptonization of synchrotron radiation internal to the jet. The model parameters calculated by us in this scenario are: magnetic field intensity B' ∼ 1 Gauss; the bulk Lorentz factor of the jet Γ[sub j] ∼ 8; and the maximum electron energy γ'[sub e, max] ∼ 4 x 10³. For these parameters the spectrum break due to synchrotron self-absorption is located at ν[sub a] ≅ 4 x 10[sup 11] Hz, which is cnnsistent with observations. [ABSTRACT FROM AUTHOR]