Reverse transcription (RT) followed by polymerase chain reaction (PCR) is the technique of choice for analysing mRNA in extremely low abundance. Real-time RT-PCR using SYBR Green I detection combines the ease and necessary exactness to be able to produce reliable as well as rapid results. To obtain high accuracy and reliability in RT and real-time PCR a highly defined calibration curve is needed. We have developed, optimised and validated an Insulin-like growth factor-1 (IGF-1) RT-PCR in the LightCycler, based on either a recombinant IGF-1 RNA (recRNA) or a recombinant IGF-1 DNA (recDNA) calibration curve. Above that, the limits, accuracy and variation of these externally standardised quantification systems were determined and compared with a native RT-PCR from liver total RNA. For the evaluation and optimisation of cDNA synthesis rate of recRNA several RNA backgrounds were tested. We conclude that external calibration curve using recDNA is a better model for the quantification of mRNA than the recRNA calibration model. This model showed higher sensitivity, exhibit a larger quantification range, had a higher reproducibility, and is more stable than the recRNA calibration curve.