Features of a probabilistic testing system that has been implemented on the "cerl" PLATO computer system are described. The key feature of the system is the manner in which an examinee responds to each test item; the examinee distributes probabilities among the alternatives of each item by positioning a small square on or within an equilateral triangle. Each of the three vertices corresponds to an item alternative. Responses are scored using an "admissible" probabilistic scoring function which enables examinees to maximize their scores in the long run if they honestly assign probabilities to alternatives. The difference between the score obtained by an examinee and the maximum possible score can be divided into two parts: (1) gain possible via a gain in knowledge about the topics covered by the test, and (2) gain possible via more appropriate assignments of probabilities to alternatives. The testing system splits the difference in this manner and reports the information to the examinee. Specific topics covered in the paper include: (1) triangle response aid; (2) scoring function; (3) performance feedback; (4) before, during, and after test administration; (5) restrictions placed on tests; (6) system TUTOR code; (7) system use; and (8) data collection. (RL)