The deoxyuridine suppression test (dUST) was used to evaluate human immunodeficiency virus type 1 positive (HIV-1) patients with low serum levels of vitamin B12 and/or low red cell folate and to assess any possible interferences of azydothymidine (AZT) in this test. The dUST was studied in 29 HIV-1 positive patients, 18 without low serum vitamin B12 or low red cell folate and 11 with low serum vitamin B12 (6 patients), low red cell folate (4 patients) and 1 case with both. The role of AZT was studied using different concentrations (0.2, 2.5 and 10 microM/ml) in 2 groups: 1 group of 5 patients with vitamin B12 and/or folate deficiency and another group consisting of 13 healthy subjects. Methotrexate (MTX)(50 micrograms/ml) was added to induce a folate megaloblastic pattern in the latter group. Results of the dUST in the HIV-1 group without low levels of serum vitamin B12 fell within the health-related reference interval values. A vitamin B12 deficiency was only detected in 1 case in the HIV-1 group with low serum vitamin B12, although a folate deficiency pattern was observed in the 4 patients with low red cell folate. In the healthy subjects AZT induced a dose-dependent decrease of the MTX-induced folate megaloblastic pattern. The pattern was also observed in the group of patients with vitamin B12 or folate deficiency, although AZT did not entirely interfere with the dUST. The effect of AZT on the dUST was attributed to a decrease in the incorporation of the isotope in the absence of deoxyuridine. The dUST is useful in differentiating vitamin B12 deficient patients from HIV-1 infected patients with low levels of serum vitamin B12.