The Transparent Online Memory Test (TOMT) introduced here has been specifically developed for online testing of word-oriented memories with parity or Hamming protection. Careful interleaving of a word-oriented and a bit-oriented test facilitates a fault coverage and a test duration comparable to the widely used March C- algorithm. Unlike similar methods TOMT actively exercises all bit cells in memory within one test period. Hence it not only detects soft errors but also functional faults and reliably prevents fault accumulation. Different variants of the basic TOMT algorithm are investigated in terms of fault coverage and test time. A prototype implementation for SRAM is introduced which-integrated into a standard processor/memory interface-autonomously performs the transparent online memory test. The trade-offs in terms of hardware overhead and memory access delay caused by this system integration are explored.