The objective of this study was to determine the effect of alendronate on the viability of canine osteosarcoma cells and nonneoplastic canine cells. The sample population was composed of canine osteosarcoma tumor cells. Osteosarcoma cells and canine fibroblasts were maintained in culture under standard conditions. The MTT assay for cell viability was performed after 24, 48, and 72 h of incubation with alendronate (0.001 to 1000 microM) or no drug (control). Plates were set up so that each concentration and the control had a sample number of 8. The optical density (OD) of each well was measured at 540 nm using an enzyme-linked immunosorbent assay microplate reader. The percent viability was determined for each concentration and for each incubation time. After 24 h of incubation of POS (parent osteosarcoma) and HMPOS cells with alendronate, there was no significant difference in mean OD at any drug concentration when compared with control samples. A significant concentration- and time-dependent reduction in mean OD of osteosarcoma cells was observed after 48 and 72 h of incubation, with alendronate concentrations ranging from 10 to 1000 microM. The lowest percent cell viability observed in treated cells was 35%. Conversely, alendronate did not significantly affect mean OD in fibroblasts, and the lowest percent cell viability observed was 76%. Our data indicate that alendronate may have the potential to inhibit canine osteosarcoma tumor growth. It will be important to determine the clinical relevance of these in vitro findings. If similar findings are observed in vivo, use of alendronate may also be indicated as an adjuvant to existing chemotherapeutic protocols.