The kinetics of crystal growth of hydroxyapatite [Ca,(PO,),OH, HAP], the thermodynamically most stable calcium phosphate phase, at 37"C, pH 7.4 and ionic strength 0.1 mol dm-, has been investigated. The kinetics results showed a surface-controlled spiral growth mechanism. The presence of 1,2-dihydroxy-1,2-bis(dihy-droxyphosphony1)ethane (DDPE) and Zn strongly inhibited t h e crystal growth of HAP. The kinetics results in the presence of each of the two inhibitors supported the suggestion that they act through adsorption and subsequent blocking of the active growth sites of the HAP seed crystals. The affinity found both for DDPE and Zn for the HAP substrate was higher by up to t h r e e orders of magnitude than the affinities reported in the literature for a number of inhibitors. In solutions in which both inhibitors existed simultaneously no synergism was found. The inhibition caused by the presence of Zn was stronger compared with that caused by DDPE, which inhibited the crystal growth of HAP more drastically as the solution supersaturation increased.