In situ viscous mass foaming with ammonium bicarbonate (NH4HCO3) as foaming agent was used for the preparation of porous hydroxyapatite ceramics. Three different particle fractions (100-300 μm, 300-500 μm, 500-1000 μm) of NH4HCO3 were used in order to evaluate the particle size effect on the ceramic microstructure. Foamed and dried green bodies were sintered at 1150°C. The porosity and microstructure of ceramics were determined with Archimedes method and scanning electron microscopy. As the NH4HCO3 particle fraction was decrease from 100-300 μm to 500-1000 μm, the open porosity increased from 42 to 54 %. In situ viscous mass foaming leads to the interconnected pore channels with wide pore size range (10-600 μm). In the present work cytotoxicity of obtained porous HA ceramics and fibroblast cell viability was investigated.