It was studied whether treatment with sulphadiazine and trimethoprim for 8 weeks affected the folate status. The therapy did not inhibit the absorption as assessed by the ³H-folic acid absorption (TRIFA) test. Plasma folic acid clearance showed that folate deficiency had not developed. Bone marrow studies revealed that the antifolic action of the drugs did not manifest itself at the cellular level. The Lactobacillus casei method should not be employed in the determinations of the serum folate concentration during the treatment because therapeutic plasma concentrations of the two drugs inhibit the growth of the test bacteria.