Porcine proinsulin, related intermediates and plasma immunoreactive insulin components have been studied by radioreceptor assay. Using the purified rat liver membrane or cultured human lymphocyte radioreceptor assay, porcine proinsulin is %5, split proinsulin 6% (54-55 split in connecting peptide) desdipeptide proinsulin 20% (deletion of amino acids 62 and 63 of connecting peptide) and desnonapeptide proinsulin 27% (deletion of amino acids 55-63 of connecting peptide) as active as porcine insulin in both assay systems; these values closely parallel the in vitro bioactivity of these preparations. In the lymphocyte radioreceptor assay the human plasma immunoreactive insulin-like component has the same potency as porcine insulin per immunoreactive unit, whereas the plasma immunoreactive proinsulin-like component is only 15% as active. Since both plasma immunoreactive components are somewhat less reactive than would be expected from puriified human insulin and proinsulin, the data suggest that both plasma components contain immunoreactive molecules that do not react in the radioreceptor assay.