Laser light scattering has been employed to determine the swimming speed distribution and the fraction of motile cells in samples of bovine spermatozoa. As predicted from theory, average trajectory velocities determined by laser light scattering were approximately four times the average translational speed estimated using light microscopy. The proportion of motile spermatozoa decreased with time at the same rate when samples were prepared in either HEPES or phosphate buffers. However, whereas the mean swimming velocity declined slowly in HEPES buffer, it dropped rapidly when phosphate buffer was used. Dilution (in the range 40 - 0.4 X 10(6) spermatozoa X ml-1) in either of these two buffers reduced the fraction of motile spermatozoa in the sample, but the mean swimming velocity of the remaining active spermatozoa was unchanged. Lowering the temperature from 37 degrees C to 15 degrees C reduced the mean swimming speed by a factor of 2-3 and the fraction of motile cells by a factor of 4-5.