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Results of an experimental investigation of adaptive beamforming in the presence of interferers are presented. Measurements at 2.05 GHz were performed using six different four-element array configurations. Data were collected in urban and suburban environments with line-of-sight (LOS) and obstructed multipath channels for mobile (vehicular) and pedestrian speeds. Interference rejection performance is determined based on the increase in mean signal-to-interference-plus-noise ratio (SINR) using adaptive beamforming at 10% and 1% outage levels. Experimental results are presented for vehicle-mounted arrays to illustrate the dependence of interference rejection on: the separation of interferer in azimuth, orientation with respect to the array axis, vehicular speed, and the array element spacing. Interference cancellation of 24 to 28 dB was measured in urban and suburban LOS channels. Vehicle mounted arrays driven at a speed of 32-40 km/hr offered 15 to 28 dB interference rejection in the urban obstructed environment. The multitarget least-squares constant modulus beamforming algorithm (MT-LSCMA) used to process the measured data provided a progressive increase in interference rejection of 16, 26, and 35.2 dB, for update intervals of 10, 5, and 2.5 ms, respectively, for the vehicle-mounted arrays in suburban LOS scenario.