Pair production and bremsstrahlung at energies >100 Bev is investigated in nuclear emulsions by studying 91 primary electron-positron pairs starting high-energy cascades and the first secondary pair. The average energy of the showers is 320 Bev. The experimental results on the total number and the energy spectrum of photons radiated by electrons ≳100 Bev show a lack of soft photons, which is in disagreement with the Bethe-Heitler theory. The experiment agrees well with the new theory given by Landau, Pomeranchuk, Migdal, and Ter-Mikaeljan. A method for obtaining the mean free path $L$ for direct pair production is presented which avoids the use of a correction for spurious tridents. A value $L={{12}_{-4\mathrm{}}}^{+7\mathrm{}}$ cm is obtained for an average electron energy of about 160 Bev. The conversion length of photons of average energy 320 Bev in nuclear emulsion is 34±5 mm, in agreement with the theoretical value of 37 mm. The distribution of the separation between the electron and the positron of the original high-energy pair is also in agreement with the theoretical distribution. This indicates that no appreciable discrepancy can exist between experiment and the theoretical cross section for the energy partition between an electron and a positron, and the probability of large energy losses by radiation. Several high-energy showers presumably produced by $\mu$ mesons and one possible case of a double pair production are described.

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