The dynamics of adsorption of bovine serum albumin (BSA) and its pentylated derivative (p-BSA) at the air-water interface was investigated through the measurements of surface pressure (Pi) and surface concentration (Gamma) via a radiotracer technique, The steady-state values of Gamma and Pi ranged from 0.8 to 1.3 mg/m(2) and from 10 to 17 mN/m, respectively, for bulk concentrations of 0.5 to 10 ppm in sodium phosphate buffer at ambient temperature. The rate of increase as well as the steady state value of Gamma were smaller whereas the rate of increase as well as the steady state value of Pi were slightly larger for p-BSA, which has a surface hydrophobicity higher than that of BSA. The observed apparent time lag for Pi, was more pronounced at lower bulk concentrations, At lower ionic strengths and at pH away from pI (the isoelectric point) of BSA, the rates of adsorption at longer times were lower, thus resulting in smaller steady-state values of Gamma. The Pi-Gamma relationship during adsorption dynamics differed from the surface equation of state obtained with the spread monolayer. The area per adsorbed protein molecule ((A) over bar) during adsorption was smaller than that for spread monolayer, indicating that the protein molecule partially unfolds during adsorption. (A) over bar for p-BSA was larger than that for BSA due to more unfolding of the p-BSA because of its lower conformational stability, as evidenced by the changes in the CD spectra of protein solution upon heating as well as a decrease in the phase transition temperature. The steady-state Pi-Gamma relationship agrees well with the isotherm obtained from the monolayer experiments, thus indicating that adsorbed BSA molecules unfold more or less completely after sufficiently long times (> 20 h), A previously developed model (G. Narsimhan and F. Uraizee, 1992, Biotech. Prog. 8, 187) was modified to better account for the electrostatic energy barrier to adsorption by postulating that the charges are uniformly distributed in an adsorbed protein layer of finite thickness. The predictions of the new model agree better with the data for native and p-BSA than the previous model, especially at low ionic strengths.