THIS communication records the occurrence of a somewhat unusual form of acute toxicity which was observed in the course of an experiment designed to investigate the possible carcinogenicity of aminopterin. As part of this investigation aminopterin (C grade) (California Corporation for Biochemical Research, Los Angeles) was examined in new-born mice. The mice used were Swiss from the colony of this laboratory that have been maintained here for 12 years and are randomly bred. Ten litters of newborn mice less than 16 h of age were treated with a single subcutaneous injection in the dorsal region of 10 µg of aminopterin dissolved in 0.05 ml. of distilled water (with 4 drops of 10 per cent potassium hydroxide added to each 5 ml. of the standard solution). The mice were distributed in 9 litters of 8 new-born animals each and 1 of 7 new-born. They were housed in plastic cages on sterilized granular cellulose bedding with the mothers.