The inadequacy of uniaxial mechanical testing data as a basis for predicting multiaxial deformation and fracture is well known. For optimal biaxial tests, computer control of simultaneous variation of loadings using hydraulic power under closed loop servocontrol is essential. Computer assistance in real time data reduction from a biaxial mechanical test is economical. Physical features of computer hardware, software, and a hydraulic biaxial system are described along with sample results illustrating cross effects in fatigue and yield surface calculations. Significance of the graphical output based on thin tube specimens of 1-in. (2.54-cm) nominal diameter, schedule 80, commercial grade polyvinyl chloride pipes, and the limitations of the present system are discussed.