Mature zygotic embryos of loblolly pine (Pinus taeda L.) were transformed by Agrobacterium tumefaciens strain LBA 4404 harbouring the plasmid pBI121 which carried the selectable marker gene, neomycin phosphotransferase II (npt II) controlled by the promoter of the nopaline synthase gene, and the uidA reporter gene, encoding beta-glucuronidase (GUS) driven by the cauliflower mosaic virus 35S promoter. Organogenic transgenic calli and transgenic regenerated plantlets were produced on selection medium containing 15 mg/L kanamycin, and confirmed by GUS histochemical staining, polymerase chain reaction (PCR), and southern blot analysis. Influences of phytohormone (BA/IBA) and antibiotics on growth and differentiation of organogenic transgenic calli were investigated. Of the phytohormone (BA/IBA) and antibiotics administered, 500 mg/L carbenicillin combined with 2 mg/L BA and 0.5 mg/L IBA (BA/IBA = 4) resulted in a 54.2% higher increase in the growth of transgenic calli as well as in the differentiation of transgenic calli, which was 45.7% more than that of control on the 6th week of culture. Claforan at 500 mg/L combined with 2 mg/L BA and 0.5 mg/L IBA resulted in a 40.8% increase in the growth of transgenic calli, and 38.7% increase in the frequency of transgenic calli forming adventitious shoots compared with the control. The growth and differentiation of transgenic calli of loblolly pine was reduced preferentially by higher BA/IBA (BA/IBA = 8), as well as high concentration of antibiotics (carbenicillin and claforan, 550 mg/L each). But it was observed that 450 mg/L and 500 mg/L carbenicillin and claforan caused an increase in growth and differentiation of transgenic calli. These results suggested that the establishment of an efficient Agrobacterium tumefaciens-mediated transformation protocol for stable integration of foreign genes into loblolly pine was also dependent on the regulation of phytohormone and antibiotic on growth and differentiation of transgenic calli. This work could be useful for the future studies of genetic transformation of conifers.