Many translational MR biomarkers derive from measurements of R1, but evidence for between-site reproducibility of R1 in small-animal MRI is lacking. Here R1 was measured by saturation-recovery in 2% agarose phantoms with five NiCl2 concentrations in 12 magnets at 5 field strengths in 11 centres on two different occasions within 1-13 days. R1 was analysed in three different regions of interest, giving 360 measurements in total. Root-mean-square repeatability and reproducibility coefficients of variation were calculated. Day-to-day repeatability was 2.3%. Between-centre reproducibility was 1.4%. Ni2+ relaxivity in 2% agarose was 0.66s-1mM-1 at 3T and 0.94s-1mM-1 at 11.7T.