A computer system has been designed which measures directly individual photomultiplier pulses of the Beckman model E analytical ultracentrifuge and converts digitized values for the pulses into an optical density (OD) as a function of radius. The system consists of a Nova computer with 8K memory, a 12 bit analog/digital converter, a Sykes cassette tape recorder for long-term storage, and a logic control and multiplexing unit which triggers the A/D converter and provides cell and sector identification for use with multihole rotors. The system can, in two revolutions of the rotor, measure the sample and reference pulses of up to five cells as well as the baseline or dard value of the photomultiplier. The radial position of the collected data is determined by counting the number of revolutions of the rotor to interpolate the distance between the outside and inside counterbalance holes. In actual operation, data are collected for 50 or 100 revolutions, sorted in terms of rotor hole and sample or reference sector, and converted into an average optical density for each cell. This single data point for each cell is then stored and the procedure is repeated for the next 100 revolutions. Calculation of OD within the computer eliminates the possibility of systematic or random error as may be present in the scanner log amplifier, holding circuits, and associated electronics. The routine for averaging the data from 100 revolutions also calculates the standard deviation for each final OD value which may be used in any analysis program for weighting the data. After the scan, the data in the computer can be stored on magnetic tape, typed out for examination, or analyzed with various fitting routines. Present developments with the system include application to high-speed and low-speed sedimentation equilibrium. The results of a sedimentation equilibrium run with myoglobin are presented to illustrate the operation and performance of the system. The useful range of OD measurements currently available with the computer system is from 0.005 to 1.6 OD.