An improved apparatus for measuring the electrical size (shape factor times volume) of particles based on the Coulter counter principle was used to characterize 110 subfertile semen specimens; size distributions were attainable in the 97 cases with a count above 1 x 10(6) spermatozoa/ml. The typical size distribution was unimodal and positively skewed. Spermatozoa maintained a constant profile during their passage through the orifice of the sizing apparatus, with an effective shape factor of 1.28. Two groups of cells were found. The larger one consisted of 68 specimens with a modal volume of 18.1 +/- 2.0 cu micron and a cell count of 27.5 +/- 9.2 x 10(6) spermatozoa/ml (mean +/- 1 SD). The smaller group contained 29 cases and had a modal volume of 6.6 +/- 0.9 cu micron and a count of 10.2 +/- 7.8 x 10(6). Samples with cell counts below 10 x 10(6) spermatozoa/ml tended to have low modal volumes (less than one-half the normal), but those above showed no particular trend.