Five spine specimens obtained at autopsy and five biopsies of sacroiliac joints from subjects with ankylosing spondylitis were submitted to microradiography and to fluorescence microscopy for detection of tetracycline. Decalcified histological sections were also prepared. Microradiography provides a link between the clinical X-ray picture and the classical decalcified section; it enables calcified cartilage, and hence the early stage of most syndesmophytes to be recognised more easily and accurately; It revealed a peculiar calcification of elastic fibers of the ligamentum flavum. Tetracycline labels showed that: in an early case of sacro-iliac arthritis, calcification of the articular cartilage might partly explain the desification of the X-ray picture; syndesmophytes were thickening at both their superficial and deep faces; thickening, lengthening and internal remodelling of the intervertebral bridges occurred together; and calcium was deposited at the end-plates as well.