This study of counting systems utilizing large plastic scintillators includes an investigation of the component parts, the γ-ray spectra obtained from large scintillators, and the application of large scintillators to total-body and low-level sample counting. Properties of the component parts investigated were: (1) light production from plastic scintillators of different manufacture; (2) the influence of crystal size and shape on light collection efficiency; and (3) the photocathode response of 5-in. and 16-in. photomultiplier tubes. Experimental spectra taken with a 16-in. diameter by 8-in. thick cylinder of plastic are shown and the results for 0.662 MeV γ-rays compared to a calculated (Monte Carlo) pulse-height distribution. The application of large plastic scintillators to total-body counting is evaluated by comparing the response of an array of 16-in. by 8-in, plastic scintillators to a single 8-in. by 4-in. NaI detector. Spectra taken with an 18-in. by 18-in. plastic well counter are presented, and data is presented on the performance of this device as a low-level sample counter.