Purified anti-spermidine monoclonal antibody was labelled with radioactive iodine by the Iodogen method and spermidine-bovine serum albumin (SPD-BSA) conjugate was used to coat polystyrene beads as solid phase antigen. The new solid phase 125I-labelled spermidine radioimmunoassay (RIA) depends on the competition between spermidine in the sample and the solid phase antigen for the limited amount of 125I-labelled monoclonal antibody. The sensitivity of this assay was 10 ng/ml higher than that of liquid RIA for spermidine with 14C-labelled spermidine. The coefficients of variation (CV) within and among batches were 4% and 13% respectively. The sample-batch capacity was increased from 20 (liquid RIA with 14C-labelled spermidine) to 150-200 by using this method. Because of its simplicity, the solid phase RIA kit is very convenient for population survey. This RIA could be used to determine spermidine in saliva for the diagnosis of precancerous lesions. In a preliminary study saliva spermidine levels in different populations were measured among 130 normal subjects, 202 esophageal epithelial hyperplasia cases treated with anti-tumor B for 5 years, 207 esophageal epithelial hyperplasia cases as control, and 55 esophageal cancer patients. The levels were 1,795 +/- 1,481, 3,470 +/- 6,981, 9,753 +/- 17,641 and 18,090 +/- 21,509 ng/ml, respectively, with the saliva spermidine levels in precancerous and cancer patients being significantly higher than that of normal subjects (P less than 0.001); the level in patients treated with anti-tumor B was significantly lower than that of controls (P less than 0.001). This decreased saliva spermidine content was coincident with the 47.3% reduction of canceration rate seen in precancerous patients after a 5-year treatment with anti-tumor B.