  The Differential Microwave Radiometers (DMR) instrument aboard the Cosmic
Background Explorer (COBE) has mapped the full microwave sky to mean
sensitivity 26 uK per 7 deg field of view. The absolute calibration is
determined to 0.7% with drifts smaller than 0.2% per year. We have analyzed
both the raw differential data and the pixelized sky maps for evidence of
contaminating sources such as solar system foregrounds, instrumental
susceptibilities, and artifacts from data recovery and processing. Most
systematic effects couple only weakly to the sky maps. The largest
uncertainties in the maps result from the instrument susceptibility to the
Earth's magnetic field, microwave emission from the Earth, and upper limits to
potential effects at the spacecraft spin period. Systematic effects in the maps
are small compared to either the noise or the celestial signal: the 95%
confidence upper limit for the pixel-pixel rms from all identified systematics
is less than 6 uK in the worst channel. A power spectrum analysis of the
(A-B)/2 difference maps shows no evidence for additional undetected systematic
effects.
