A RIA with a minimal sensitivity of 0.5 ng protein was developed for the measurement of bovine secretory protein-I (SP-I). With this assay and a previously established RIA for parathormone, the secretion and cell content of SP-I and parathormone were determined in dispersed bovine parathyroid cells. SP-I and parathormone were secreted in linear fashion over a 2-h period. The net secretion of both of these proteins diminished progressively as the concentration of calcium was raised from 0.25 mM to 3.0 mM. The molar ratio for the secreted proteins and those remaining in the cell varied from experiment to experiment but on average was 0.70 ± 0.07 for the secreted proteins and 0.47 ± 0.03 for the cellular proteins. Possible explanations for the difference in the ratio of SP-I to parathormone between cellular and secreted proteins include 1) a preferential secretion of SP-I; 2) a preferential intracellular degradation of SP-I; 3) a preferential postsecretory degradation of parathormone, or 4) differential affinities of potential fragments of either or both proteins for their antisera. These results suggest that SP-I and parathormone bear close but not identical metabolic and secretory fates.