The relaxation of 1H nuclei due to their interaction with quadrupolar 14N nuclei in gel structures is measured using fast field-cycling NMR. This phenomenon called quadrupolar dips has been reported in different 1H-14N bond-rich species. In this study, we have studied quadrupolar dips in fibrin, an insoluble protein that is the core matrix of thrombi. Fibrin was formed by the addition of thrombin to fibrinogen in 0.2% agarose gel. T1-dispersion curves were measured using fast field-cycling NMR relaxometry, over the field range of 1.53.5 MHz (proton Larmor frequency), and were analyzed using a curve-fitting algorithm. A linear increase of signal amplitude with increasing fibrin concentration was observed. This agrees with the current theory that predicts a linear relationship of signal amplitude with the concentration of contributing 14N spins in the sample. Interestingly, fibrin formation gave rise to the signal, regardless of crosslinking induced by the transglutaminase factor XIIIa. To investigate the effect of proteins that might be trapped in the thrombi in vivo, the plasma protein albumin was added to the fibrin gel, and an increase in the quadrupolar signal amplitude was observed. This study can potentially be useful for thrombi classification by fast field-cycling MRI techniques. Magn Reson Med, 2012. (c) 2011 Wiley Periodicals, Inc.