A rabbit antiserum specific for LDH-X, the spermatozoal form of mouse lactate dehydrogenase, was prepared. This antiserum had no effect on fertility of female mice when injected before or after insemination. Similarly, there was no toxicity to the embryo when high concentrations of the antiserum were added to cultures of 2-cell and 8- to 16-cell embryos. There was, however, a moderate inhibitory effect on fertilization in vitro, which may be attributable to a direct action of antiserum to LDH-X on sperm.