1. The passive immunisation of mice and rats to lens was undertaken to study the effects of the antibodies on embryos. Rat lens, sheep lens, ox lens, and ox vitreous humour were used as antigens and antisera prepared. 2. Only lens antisera that gave precipitin reactions were employed for injection. These different sera all gave almost equally definite precipitin reactions with a normal saline extract of either mouse, rat, sheep, or ox lens. 3. The antisera were injected chiefly into female mice, the majority of which could be expected to be pregnant. The results differed greatly. The anti-rat lens serum was very toxic and the mice which received injections of this gave an average litter of 1.1 within the expected time; the sheep and ox antiserum group averaged 2.9; while the control group injected with anti-vitreous humour produced an average of 5 per litter. It would therefore seem that, as antigens, these different species lens are not so organ specific as the precipitin test would indicate. The relation of the rat to the mouse could account for the toxicity of the anti-rat lens serum for embryonic mice. 4. The eyes of the surviving F1 were normal. An F2 generation by brother x sister matings was also normal. 5. The active immunisation of female rats and mice to sheep's lens was attempted, but the litters born under treatment were of fair average numbers and were normal in every respect. If antibodies against sheep lens had been generated in the bodies of the female rats, as one could expect, then these had no apparent effect on the embryo. 6. Female rats were injected with rat lens at intervals for several weeks, until they became pregnant. The F1 were normal. From the breeding results there was nothing to indicate that antibodies toxic for embryos had been generated. 7. In these experiments it is the injection of anti-rat serum into mice that has the most bearing on the work done by Guyer and Smith with rabbits. They found that anti-rabbit lens serum was very toxic for embryo rabbits. Fortunately they were enabled to rear a number of the affected young which had defective eyes due to the specific influence of the antiserum. But with mice it is difficult to rear defectives, hence it is quite possible that of the many embryos that died, some were effected in a specific manner by the anti-lens serum