The microstructure of the F82H steel that is a candidate for the future fusion reactor first wall is investigated. Protons as well as fission neutrons are used to simulate the irradiation effects of the fusion environment. The differences in the He production rates between the two types of irradiation may lead to differences in the irradiation damage of the F82H microstructure. This paper presents a transmission electron microscopy (TEM) study for three different irradiation conditions: (i) in the PIREX facility with 590 MeV protons, (ii) with fission neutrons at the research reactor in Petten and (iii) with 590 MeV protons in PIREX followed by fission neutrons at the reactor in Studsvik. The latter experiment allows us to vary the He production rate for a given irradiation type. Total doses ranged from 0.3 to 10 dpa and temperatures ranged from room temperature to 310 degreesC. (C) 2000 Elsevier Science B.V. All rights reserved.