We report the first results of a millimeter search for cosmic microwave background (CMB) anisotropies, carried out by means of a balloon experiment (ARGO 1993). The experiment has observed 63 independent sky fields in Hercules, with an angular resolution of 52' FWHM, in four photometric bands at 2.0, 1.2, 0.8, and 0.5 mm. A statistically significant sky signal has been observed above detector noise at 2.0 and 1.2 mm. The signal is spectrally consistent with CMB anisotropies, while reasonable local sources of anisotropy are excluded. For temperature fluctuations with a coherence angle of theta(c) = 30' we get 1.4 x 10(-5) < DELTAT/T < 3.0 x 10(-5) (90% confidence interval, calibration error included). The data are also compared to theoretical models and are found to be in agreement with the CMB anisotropy detected by COBE