Currently used methods for plasma cAMP measurements are either tedious (chromatographic preparation of sample) or potentially inaccurate (direct assay of plasma samples). A rapid, simple, and accurate competitive binding assay for plasma cAMP, which does not require chromatographic preparation of the sample, has been developed. This procedure prevents destruction of plasma cAMP by utilizing both theophylline and EDTA in the collection of the blood sample. Human plasma contains variable amounts of cAMP-binding activity which interfere with the measurement of cAMP by the standard competitive binding assay. Our assay procedure removes this binding activity by precipitation of plasma proteins with perchloric acid. The normal fasting value (+/- SD) of plasma cAMP using this technique is 17.6 +/- 4.3 pmol/ml, which is identical to values obtained by methods utilizing chromatographic purification of samples (18.3 +/- 3.0). The fasting plasma cAMP of patients with hyperparathyroidism is normal (16.2 +/- 3.4), but patients with maturity-onset diabetes mellitus have fasting values significantly below normal (12.3 +/- 2.4).