In three patients a mechanically well-fixed Mathys Ceros 80 (Ha) hydroxyapatite-coated acetabular component was revised 2, 5 and 13 months after total hip replacement due to component malposition. In each case there was a thin cellular connective tissue membrane between hydroxyapatite and bone, the main cell type being fibroblast with only occasional giant cells. Immunohistological analysis revealed some MHC locus II antigen positive cells that were identified as monocytes. No interleukin-2 receptor positive cells were found. Under clinical cyclic loading conditions there does not seem to be chemical fixation or bony ingrowth into the hydroxyapatite coated prosthesis component. In human lymphocyte cultures, hydroxyapatite (Interpore 200, particle diameters 15-40 microns) did not cause an increase in lymphocyte DNA synthesis as assessed by the 3H-thymidine incorporation method on culture days 1, 3 and 5. As analysed with lymphocyte activation markers, the hydroxyapatite-dependent expression of MHC locus II antigen was modest and differed significantly (P less than 0.05) from that in culture medium only on day 3. Hydroxyapatite induced only a slight interleukin-2 receptor expression that did not differ from culture medium on days 1, 3 and 5. CD4 and CD8 positive lymphocytes as well as monocytes were not seen attached to hydroxyapatite particles during the culture days. Our findings suggest that hydroxyapatite is an immunologically inert implant material.