The proton spin-lattice relaxation rate (1/T-1) in malignant and non-malignant ascites was measured with an FT NMR spectrometer operating at 60 MHz. The mean relaxation rate in non-malignant ascites was significantly smaller than that of malignant ascites. However, the 1/T-1 of malignant ascites overlaps with that of non-malignant ascites over all concentrations of total protein (TP) in samples. The 1/T-1 in non-malignant ascites correlates strongly with TP, whilst the 1/T-1 in malignant ascites shows only a moderate correlation. T-1 measurements before and after addition of ascorbic acid (reductant) suggest that there is a small paramagnetic contribution of ions to the 1/T-1 in malignant ascites. The least-squares fitting of 1/T-1 versus TP for non-malignant data gives a linear relationship, and suggests that the T-1 mechanism in non-malignant ascites is caused by a fast chemical exchange of water molecules between protein-bound water and free water.