Enhancement of the effective electron mass by spin fluctuations (SF's) has been studied in a-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Zr}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ (27%\ensuremath{\leqslant}x\ensuremath{\leqslant}39%) by low-temperature calorimetry. Previous magnetization measurements on paramagnetic a-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Zr}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ are also presented and analyzed to extract the ground-state susceptibility \ensuremath{\chi}(T=0). The low spin fluctuation temperature of \ensuremath{\sim}7 K and the stable amorphous structure make it possible to unambiguously distinguish between magnetic and nonmagnetic contributions to the electronic specific heat. The SF contribution to the specific heat shows a clear maximum around the critical composition for the onset of magnetic order (${\mathit{x}}_{\mathit{c}}$\ensuremath{\sim} 37%), with the mass enhancement parameter ${\ensuremath{\lambda}}_{\mathrm{SF}}$ reaching \ensuremath{\sim}1. Complete quenching of SF's by both temperature and an external magnetic field have been observed. As a result of its low characteristic temperature (${\mathit{T}}_{\mathrm{SF}}$\ensuremath{\sim}7 K), a-${\mathrm{Fe}}_{36}$${\mathrm{Zr}}_{64}$ shows the clearest evidence to date of the effects of SF's on electronic properties. \textcopyright{} 1996 The American Physical Society.