X-ray and Hα observations of an erupting filament, discussed herein, and other observations of the associated flare on 1980 May 21, suggest that an erupting filament played a major role in the X-ray flare. While Antonucci et al. (1985) analyzed the May 21 flare as one of the best cases of chromospheric evaporation, the possible contribution from X-ray emitting erupting plasma has been ignored. We show that pre-heated plasma existed and may have contributed part of the blue-shifted X-ray emission observed in the Ca xix line, which was formerly attributed solely to chromospheric evaporation. Thus it remains an open question - in two-ribbon flares in particular - just how important chromospheric evaporation is in flare dynamics.