In earlier studies it was found that the effects of urethane could be reversed by folic acid and thymidine but not by uridine. It was further shown that urethane-treated chick embryos were unable to convert serine to glycine, homocysteine to methionine and uridine to thymidine, all of which are folate-dependent reactions. Due to the sensitivity of folic acid auxotrophs towards small amounts of folates present in the biological material, the folic acid contents of urethane-treated and untreated chick embryos are studied, using microbial assay. It is found that there is a statistically significant reduction in the total folate content of chick embryos treated with urethane as compared to those from the untreated controls. It is concluded that urethane has an antifolate action.