The interface mechanics and bone growth into porous Co-Cr-Mo alloy implants were evaluated. Three mean pore sizes (155, 235, and 350 microns) were studied for implants fabricated with one, two, and three layers of spherical powder particles and totally porous implants. All implants had a pore volume of 38%-40%. Ten implants were inserted transcortically in the femora of six adult mongrel dogs and were allowed to remain in situ for a period of 12 weeks. Postretrieval mechanical testing and histologic and microradiographic analyses were used to evaluate the implant systems. A statistically significant increase in interface shear strength (p less than .05) was determined as the number of powder particle layers was increased from one and two layers to three layers. However, a decrease in interface strength was determined for totally porous implants (seven to 12 particle layers) below the value for the single-porous-layer implants. Pore size, in the range investigated, was not found to influence the interface attachment strength. Neither pore size nor porous layer thickness was found to affect interface stiffness. Histologic and microradiographic sections revealed extensive mineralized bone growth deep into the pores of all implant types and often extending to the core of the one-, two- and three-layered porous-coated implants. Both mature haversian bone and less mature woven bone were found within the porous structure. Extensive but incomplete bone infiltration was found in the totally porous implants, with the remainder of the porosity filled with macrophage-laden connective tissue. No difference in the histologic response was observed as a function of pore size in any of the implant systems. Active bone labels were present at the 12-week time period, indicating continued bone remodeling.