1 Cross-reactivity of ergot alkaloids with an antiserum produced against lysergic acid conjugated with human serum, albumin was utilized to develop a radioimmunoassay for ergotamine, dihydroergotamine, dihydroergotoxine, ergometrine and methylergometrine in biological fluids. The antisera showed no cross-reactivity with simpler indole structures. 2 A procedure for extraction and concentration of alkaloids in biological fluids was developed. 3 The assay is sensitive for 1.8 ng/ml ergotamine, 1.5 ng/ml dihydroergotamine, 2.2 ng/ml dihydroergotoxine, 0.7 ng/ml ergotmetrine and 0.5 ng/ml methylergometrine. 4 The assay is sufficiently sensitive to permit the measurement of urine and plasma ergot alkaloid levels and it is suitable for determination in cases where a known ergot alkaloid is used.