Any fully-ionized collisionless plasma with finite random particle velocities contains electric and magnetic field fluctuations which are of three different types: weakly damped, weakly propagating or aperiodic. The kinetics of these fluctuations in general unmagnetized plasmas is governed by the competition of spontaneous emission, absorption and stimulated emission processes. The generalized Kirchhoff laws for both collective and non-collective fluctuations are derived, which in stationary plasmas provide the equilibrium energy densities of electromagnetic fluctuations by the ratio of the respective spontaneous emission and true absorption coefficients. The equilibrium energy densities of aperiodic transverse collective electric and magnetic fluctuations in an isotropic thermal electron–proton plasma of density ne is calculated as and , where g and βe denote the plasma parameter and the thermal electron velocity in units of the speed of light. For densities and temperatures of the reionized early intergalactic medium ∣δB∣ = 6 · 10−18 G and ∣δE∣ = 2 · 10−16 G result.