Available bioassays for corticotrophin releasing factor (CRF) (McCann & Haberland, 1959; De Wied, 1961; Arimura, Saito & Schally, 1967; Hiroshige, Kunita, Yoshimura & Itoh, 1968) are unsuitable for routine purposes because of their technical complexity and lack of sensitivity. Assays employing hemipituitaries in vitro (Saffran & Schally, 1955; Sadow, Penn & Knight, 1972) suffer from the inability of substances to diffuse freely into and out of those cells which are shielded from the bathing medium. The use of isolated pituitary cells overcomes this disadvantage and the batch-type incubation of cells has been described by Portanova, Smith & Sayers (1970), Portanova (1972) and Portanova & Sayers (1973). Corticotrophs, however, contain large stores of corticotrophin (ACTH) and consequently their lysis and death during incubation can lead to high background values. The recently developed technique of cell-column perfusion (Lowry & McMartin, 1974) for the measurement of releasing factor activity offers the advantage that