An efficient procedure is described for the transformation of the monocotyledonous Alstroemeria by Agrobacterium tumefaciens via callus regeneration. Calli derived from ovules were co-cultivated with A. tumefaciens strains EHA101 and LBA4404, which harbored the binary vector plasmids pIG121Hm and pTOK233, respectively. These plasmids contain the beta-glucuronidase gene ( gusA) as a reporter gene and the hygromycin phosphotransferase and neomycin phosphotransferase II ( nptII) genes as selective markers. Inoculated calli were first plated for 4 weeks on medium containing cefotaxime to eliminate bacteria, following which time transformed cells were selected on medium that contained 20 mg/l hygromycin. A histochemical assay for GUS activity revealed that hygromycin-based selection was completed after 8 weeks. The integration of the T-DNA of pIG121Hm and pTOK233 into the genome of the cells was confirmed by PCR analysis. Efficient shoot regeneration from the transformed calli was observed on half-strength MS medium supplemented with 0.5 mg/l naphthaleneacetic acid and 0.5 mg/l benzyladenine after about 5 months of culture. The presence of the gusA and nptII genes in the genomic DNA of regenerated plants was detected by means of PCR and PCR-Southern hybridization, and the expression of these transgenes was verified by reverse transcription-PCR.