Experiments dealing with levels of alpha-6-desoxy-5-oxytetracycline (doxycycline, DOOTC) in human organ tissue after i.v. applications of 2 times 200 mg have been conducted. 100 biopsies yielding 371 samples (generally 4 samples at 70 +/- 14 mg per specimen) were tested against Bacillus subtilis strain ATCC 6633 using the agar diffusion method (test plate: dst-agar Oxoid CM 261, pH 6.3-6.4, basic layer without and overlay with spores). Taking into account the manifold sources of error in diffusion tests, the results should be understood only as a basic outline without overestimating the tabulated figures of the antibiotic contents in tissue. Most of the organs showed DOOTC levels which were distinctly higher than the minimal inhibiting concentration effective against DOOTC susceptible bacteria.