SUMMARYSensitive direct (non‐extraction) radioimmunoassays for assay of progesterone (Po) in saliva and capillary blood dried on filter paper have been developed to facilitate collection of specimens for the assessment of luteal function in the investigation of infertility. The antiserum had been raised to a Po I lα‐hemisuccinyl conjugate and the assays employ Po 11α‐glucurony 1‐[25I] tyramine as tracer with separation by solid phase double antibody. Po levels in saliva and capillary blood were compared with those in serum in matched specimens collected daily throughout the menstrual cycle from six female volunteers. The correlation between blood (y) and serum (x) Po levels (y= 0.439x‐2.98,r=0.98) was superior to that between salivary (y) and serum levels (x) (y=0.0637x+0.020.r= 0.89). Blood spot and serum Po levels were uniformly undetectable throughout the follicular phase, whereas in 2 out of 72 saliva specimens Po was inappropriately raised (60 and 120 pmol/l) possibly reflecting contamination of the specimens. Salivary and blood progesterone assays both lack adequate sensitivity to measure Po reliably in early or late luteal phases when serum levels are<12 nmol/l but are well suited to assess luteal function during the mid‐luteal phase of the cycle (4–10 d before menses) when serum levels in the six subjects ranged from 22–84 nmol/l, with corresponding blood levels of 6.2–35 nmol/l and saliva levels of 130–620 pmol/l. We conclude that blood spot and saliva Po levels both correlate well with Po levels in serum, but that blood spot Po lev