Background The availability of data in the medical literature for the T-2 relaxivities of the Gd-based contrast agents (GBCAs) is limited. A comprehensive comparison between the agents available commercially (other than in Europe) is lacking, with no data available that most closely reflect the clinic, which is in human whole blood at body temperature. Purpose To complement the existing literature by determining T-2 relaxivity data for eight GBCAs in vitro. Material and Methods The relaxivities of eight GBCAs diluted in human whole blood at 1.5 and 3 T were determined at 37 +/- 0.5 degrees C. Gd was in the range of 0-4 mM. Multi-echo sequences with variable echo times were acquired using a phantom containing a dilution series with each agent, and SigmaPlot 12.0 was used to calculate the R-2 relaxation rate and finally r(2). Statistical comparisons between agents and field strengths were conducted. Results The relationship between R-2 vs. Gd was observed to be linear at 1.5 and 3 T, with a mild increase in r(2) from 1.5 to 3 T for all GBCAs. T-2 relaxivity data were compared with prior results. The GBCAs are closely clustered into two groups, with higher r(2) noted for the two lipophilic (those with partial hepatobiliary excretion) compounds. Conclusion The r(2) values at 1.5 and 3 T, determined for the eight GBCAs still clinically available (other than in Europe), provide a definitive baseline for future evaluations, including theoretical calculations of signal intensity and their clinical impact on T2-weighted scans.