The viability of porcine collagenase-prepared islet preparations (n = 16) was classified by 31P-NMR spectroscopy, staining by neutral red and trypan blue, and in vitro insulin secretion following glucose challenge. Vital islets exhibited a phosphate diester/phosphate monoester (PDE/PME) ratio of 0.5-0.9, a staining score of 18-30 and an insulin secretion responding well to glucose challenge. Damaged islets performed at a PDE/PME of 0.2-0.49 and a staining score of 9-17 and necrotic islets had 0.0-0.49 and a staining score of 9-17 and necrotic islets had 0.0-0.19 and 0-8, respectively. The islets of the latter two groups did not adequately respond to glucose. The in vivo function following autotransplantation of these islets into the spleen was investigated in five recipients of more than 3000/kg vital islets of which 4 expressed daily normoglycemia (< 200 mg%), normalized intravenous glucose tolerance (K = -2.21), and a prolonged survival (mean +/- SD) of 167 +/- 12 days compared to five recipients of > 3000/kg damaged islets (K = -0.814) (P = 0.0017) and a survival of 86 +/- 21 days (P = 0.0096). It is suggested that 31P-NMR spectroscopy is a valuable and practical method to predict islet graft viability prior to transplantation in order to assure good graft function in the recipient.