Objectives: Calculation of accurate T1 relaxivity (r(1)) values for gadolinium-based magnetic resonance contrast agents (GBCAs) is a complex process. As such, often referenced r(1) values for the GBCAs at 1.5 T, 3 T, and 7 T are based on measurements obtained in media that are not clinically relevant, derived from only a small number of concentrations, or available for only a limited number of GBCAs. This study derives the r(1) values of the 8 commercially available GBCAs in human whole blood at 1.5 T, 3 T, and 7 T. Materials and Methods: Eight GBCAs were serially diluted in human whole blood, at 7 concentrations from 0.0625 to 4 mM. A custom-built phantom held the dilutions in air-tight cylindrical tubes maintained at 37 +/- 0.5 degrees C by a heat-circulating system. Images were acquired using inversion recovery sequences with inversion times from 30 milliseconds to 10 seconds at 1.5 T and 3 T as well as 60 milliseconds to 5 seconds at 7 T. A custom MATLAB program was used to automate signal intensity measurements from the images acquired of the phantom. SigmaPlot was used to calculate T1 relaxation times and, finally, r(1). Results: Measured r(1) values in units of s(-1).mM(-1) at 1.5 T (3 T/7 T) were 3.9 +/- 0.2 (3.4 +/- 0.4/2.8 +/- 0.4) for Gd-DOTA, 4.6 +/- 0.2 (4.5 +/- 0.3/4.2 +/- 0.3) for Gd-DO3A-butrol, 4.3 +/- 0.4 (3.8 +/- 0.2/3.1 +/- 0.4) for Gd-DTPA, 6.2 +/- 0.5 (5.4 +/- 0.3/4.7 +/- 0.1) for Gd-BOPTA, 4.5 +/- 0.1 (3.9 +/- 0.2/3.7 +/- 0.2) for Gd-DTPA-BMA, 4.4 +/- 0.2 (4.2 +/- 0.2/4.3 +/- 0.2) for Gd-DTPA-BMEA, 7.2 +/- 0.2 (5.5 +/- 0.3/4.9 +/- 0.1) for Gd-EOB-DTPA, and 4.4 +/- 0.6 (3.5 +/- 0.6/3.4 +/- 0.1) for Gd-HP-DO3A. The agents can be stratified by relaxivity, with a significant additional dependency on field strength. Conclusions: This report quantifies, for the first time, T1 relaxivity for all 8 gadolinium chelates in common clinical use worldwide, at current relevant field strengths, in human whole blood at physiological temperature (37 degrees C). The measured r(1) values differ to a small degree from previously published values, where such comparisons exist, with the current r(1) measurements being that most relevant to clinical practice. The macrocyclic agents, with the exception of Gd-DO3A-butrol, have slightly lower r(1) values when compared with the 2 much less stable linear agents, Gd-DTPA-BMA and Gd-DTPA-BMEA. The 2 agents with hepatobiliary excretion, Gd-EOB-DTPA and Gd-BOPTA, have, at 1.5 and 3 T, substantially higher r(1) values than all other agents.