The mouse strains C3H/Bi McL and C57BL/McL were shown to have markedly different spermatozoa, and, by combining four different sperm dimensions by means of a discriminant function, it was possible to identify individual spermatozoa from the two strains with a calculated misclassification probability of 2.1%. Hybrids had intermediate sperm discriminant values. The sperm from five C3HC57 chimeras were characterized using the discriminant function, and it was found that one chimera had C57-like sperm, another had C3H-like sperm, while each of the other three had sperm of both phenotypes. In no case did the sperm populations of chimeras resemble those of hybrids. A detailed analysis of the sperm dimensions of the chimeras showed that the differences between the two populations of sperm and the variances of these populations are the same as for C3H and C57 sperm populations from pure strain mice. These observations imply that sperm dimensions are determined at the level of individual spermatozoa but do not differentiate between intrinsic (germ line) and extrinsic (Sertoli cell) control. However, the proportions of C3H-type and C57-type sperm in the chimeras were found to be correlated with the proportions of C3H-derived and C57-derived offspring but not with the proportions of C3H and C57 cells in somatic tissues. It is argued that the differences in sperm dimensions between the two strains are due to genes expressed through the germ line.