To be pharmacologically active, i.v. prednisolone disodium phosphate (prednisolone phosphate), prednisolone sodium tetrahydrophthalate (prednisolone phthalate), and oral prednisone have to be converted in vivo into prednisolone. The purpose of the present study was to establish whether these agents are functionally equivalent when given in equimolar doses. The peak concentrations of total and unbound prednisolone assessed by HPLC and equilibrium dialysis in five normal volunteers were five to 12 times higher after i.v. prednisolone phosphate than after i.v. prednisolone phthalate or oral prednisone (p less than 0.001). The area under the plasma concentration vs time curves (AUC) of total and unbound prednisolone was higher after i.v. prednisolone phosphate than after oral prednisone or i.v. prednisolone phthalate (p less than 0.001). The immunosuppressive activity of the plasma samples was determined as the percentage inhibition of the mixed lymphocyte reaction, and the area under the inhibition vs time curve of the MLR (AUIC), a measure of immunosuppressive capacity over time, was calculated. After i.v. prednisolone phthalate or oral prednisone, the mean AUC of unbound prednisolone were 45 and 61% of that obtained after i.v. prednisolone phosphate, and the corresponding AUIC were 76 and 68%, respectively, indicating lower systemic availability of prednisolone and less of an immunosuppressive effect in plasma after i.v. prednisolone phthalate or oral prednisone than after i.v. prednisolone phosphate. These differences in biologic activity of the plasma were corroborated by determining ex vivo the cell cycle progression within the G1 phase, and the interleukin 2 (IL 2) release after the activation of peripheral lymphocytes obtained at various time points after administration of the three steroids. After steroid doses corresponding to 1 mg/kg of prednisolone, a transient inhibition of 80% of IL 2 production was observed with all three glucocorticoids given, whereas the maximal inhibition of the IL 2 production was 60, 30, and 22% after a low dose (0.1 mg/kg) of prednisolone phosphate, prednisone, or prednisolone phthalate, respectively. After the low dose, only minimal changes in the percentages of the OKT4+ and OKT8+ cells were detectable, whereas after the high dose of all three steroids the ratio of OKT4+ to the OKT8+ cells dropped, the decline being more pronounced after prednisolone phosphate than after prednisolone phthalate or prednisone.(ABSTRACT TRUNCATED AT 400 WORDS)