Reverse transcriptase-PCR (RT-PCR) amplification of mRNA is often the only technique able to detect expression of cytokine mRNA in small samples. The aim of this work was to investigate the utility of a non-competitive RT-PCR which used external standards to quantitate TNF-alpha mRNA in liver biopsy specimens from liver transplant patients. It involved removal of aliquots from the PCR reaction at successive cycles, followed by dot-blotting of the samples onto nylon membrane and hybridization with a radioactively-labelled internal probe. Phosphorimage analysis of the labelled membranes allowed quantitation of the relative amount of PCR product at successive cycles. Plots of log(counts) versus cycle number showed straight lines in the exponential phase of amplification. The slopes of these lines showed the efficiency of amplification, which ranged from 76 to 87% for liver biopsy samples. Estimation of liver biopsy levels of TNF-alpha in two separate PCR amplifications showed low inter-assay variability (r2 = 0.98). Comparison of two separate cDNA syntheses also showed good correlation (r2 = 0.81, P < 0.0001), although not as good as for the PCR alone. This shows that variation in efficiency of cDNA synthesis is likely to contribute as much or more to variability of the analysis as variations in PCR amplification.