1. To determine any interaction between omeprazole and cyclosporine A (CsA) 10 male patients with 1-7 year renal transplants and stable renal function, participated in this randomised blind cross-over trial with omeprazole, 20 mg, and placebo treatment once daily for 2 consecutive weeks each. 2. Blood samples for measurement of trough concentrations of CsA were obtained twice a week during the 4 study weeks, and during the 2 weeks before and after the study. Unchanged CsA concentrations were measured by h.p.l.c. and using a monoclonal antibody r.i.a., and drug plus metabolites were measured by a polyclonal antibody f.p.i.a. In addition, one of the metabolites, M17, was assayed separately by h.p.l.c. 3. The mean whole blood trough CsA concentration during omeprazole treatment was 102 (95% confidence interval, 84-122) micrograms l-1 determined by h.p.l.c. and 81 (65-100) micrograms l-1 determined by r.i.a. Corresponding values during placebo treatment were 100 (79-127) micrograms l-1 and 95 (75-120) micrograms l-1. The ratios between omeprazole and placebo treatments were 1.01 (0.84-1.22) (h.p.l.c.) and 0.85 (0.67-1.08) (r.i.a.). Assuming that a change of < 30% in CsA blood concentrations is of no clinical significance, these results show that there was no clinically or statistically significant influence of omeprazole on CsA concentrations. Neither CsA concentrations determined by f.p.i.a. nor determination of M17 by h.p.l.c. indicated any effect of omeprazole on the metabolism of CsA. 4. It is concluded that omeprazole (20 mg daily) does not significantly interfere with CsA metabolism in stabilised renal transplant patients and may be used safely without extra monitoring of blood CsA concentrations.