A sensitive enzyme-linked, indirect immunosorbent assay (ELISA) for the determination of guanosine 3',5'-cyclic monophosphate (cGMP) in glial cells is described. The assay uses an antiserum produced against succinylated cGMP and is based on the competition between endogenous cGMP and a fixed amount of immobilized antigen. The antibody exhibited a high degree of specificity with negligible cross reactivity with other nucleotides or related compounds. The standard curve, linearized using a logit-log transformation, had an operating range of 1 fmol/50 microl to 5 pmol/50 microl. The sensitivity of the assay was significantly increased by acetylation of standards and samples. Recoveries of cGMP from samples of cultured cells ranged from 85% to 105% with a mean recovery of 97% +/- 7%. Levels of cGMP measured with the ELISA were in agreement with the corresponding values obtained using radioimmunoassay. The present method provides for a cheap, sensitive and simple alternative to the commercially available cGMP assays.