The sequential tests of MIL-STD 781-B are based upon the accumulated test hours of a number of test units. The number of test units, however, is not specified and may be varied to suit the testing agency. The problem here is to find the optimum number of units to place on test based upon a hypothetical cost model. The cost of testing is considered to be a composite of those costs which depend upon the number of units employed and of those depending upon the duration of the test facility participation. A cost model of these fixed and variable costs is illustrated for Test Plan I of 781-B under five assumed levels of mean time before failure and for four unit cost-to-facility cost relationships. Tables of the composite costs for the nine other test plans of 781-Bareavailable upon request from the author.