The cyanobacterium Spirulina platensis was tested for sensitivity to 6 antibiotics: kanamycin (Km), neomycin (Neo), geneticin (G418), ampicillin (Ap), chloramphenicol (Cm), hygromycin (Hm) and to 1 herbicide: L-phosphinothricin (PPT), all of which are frequently applied as selectable markers in plant gene engineering. Growth tests, carried out at different concentrations of the agents above, showed that S. platensis was sensitive to Hm, Cm and PPT, while highly resistant to Km, Neo, G418 and Ap, indicating that Hm, Cm and PPT were potent inhibitors of S. platensis growth and able to be employed as selectable markers for further transformation studies for this alga.