1. Rabbit antisera against arginine-vasopressin (AVP) were evaluated for sensitivity and specificity in a radioimmunoassay based on the extraction of AVP from plasma and urine on to Florisil.

2. Comparison of the immunoreactivity of AVP with analogues showed that one antiserum (R2) reacted principally with the hexapeptide ring and another (R4) bound to the tripeptide tail and was reactive with some reduction and hydrolytic products of the native peptide.

3. The minimum amount of AVP measurable in the radioimmunoassay was 1 pg. The extraction of AVP from plasma and urine gave a recovery of 93% (sd 5%). A plasma sample repeatedly assayed with R2 gave a value of 1·4 ng/l (sd 0·2, n = 12).

4. The antiserum specific for the hexapeptide ring (R2) showed that in normal subjects AVP concentration ranged from 3·2 ± 2·52 ng/l after dehydration to 0·16 ± 0·1 ng/l after water loading.

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