Colloidal calcium hydroxyapatite (Ca-10(PO4)(6)(OH)(2); HAP) particles doped with Ti(IV) ions in different atomic ratios, Ti/(Ca + Ti) = X-Ti, by a coprecipitation method were characterized by TEM, U-V, FTIR, XPS, and ICP-AES. The photocatalytic activity of the modified HAP particles was examined by decomposition of acetaldehyde and albumin and a bactericidal test by colon bacillus. Ca(H) of HAP was one-to-one substituted by Ti(IV) at X-Ti less than or equal to 0.1. When doped at X-Ti > 0.1, irregular particles of amorphous titanium phosphate were formed besides long rectangular particles of HAP. X-Ti of the surface phase of the particles was much less than that of the whole particle; Ti(IV) is less contained in the surface phase than in the bulk one. A UV beam was absorbed by Ti(IV)-modified HAP particles but not by the unmodified particles. The decomposition of acetaldehyde and albumin by Ti(IV)-doped particles was found under UV irradiation, while the unmodified HAP particles were inactive for the decomposition of both materials. Further, different from TiO2, Ti(IV)-doped HAP showed a bactericidal function in the dark.