The interaction between the main components of the new glycopeptide antibiotic teicoplanin, A2-2, A2-3, A2-4, A2-5 and A3-1, and human serum albumin has been studied in vitro by equilibrium dialysis (pH 7.4, 37 degrees C). From Scatchard analysis of the data, the calculated association constants (Ka) were: A2-2, 2.47 X 10(4), A2-3, 2.86 X 10(4), A2-4, 2.95 X 10(4) and A2-5, 3.87 X 10(4) mol.l-1. The number of binding sites per albumin molecule ranged between 1.23 to 1.31. A3-1 had a lower affinity with a Ka of about 5 X 10(3) mol.l-1. Extrapolated to the in vivo situation, the data suggested that about 90-95% of A2 components will be bound to serum albumin, and about 68-72% of A3-1. The in vitro findings were confirmed by a pharmacokinetic study in volunteers given [14C] teicoplanin i.v., in whom the fraction of teicoplanin bound to serum protein ranged between 87.6 and 90.8%.