Homofolic acid (HF) was found to inhibit the growth of Lactobacillus casei ATCC 7469 in 100 ng/ml or higher concentrations in the presence of folic acid (PGA) on short (24-48 h) incubation. HF blocked the uptake of 3HPGA into L. casei cell suspensions. On prolonged (3-5 days) incubation HF (greater than 10 ng/ml) promoted growth of L. casei both in the presence and absence of PGA. No growth was obtained when both PGA and HF were omitted. Thus, HF was apparently biotransformed from an inhibitory analog into a growth-promoting HF derivative. Indeed, sterile filtrates from HF-grown cultures supported growth of L. casei in a folate-free medium on short incubation, supporting the notion that a folate-like active compound was formed from HF on prolonged incubation with L. casei. In the presence of homopteorate (10-60 micrograms/ml) no transformation of HF into folate-active compounds took place.