Objective: To evaluate a remineralizing fluid for its influence on in vitro caries formation and progression. Method and Materials: Caries-free teeth (n = 12) were sectioned into 3 segments. Each segment was assigned to a treatment group: (1) remineralizing fluid for 2 minutes; (2) 1.23% acidulated phosphate fluoride (APF) gel for 1 minute; (3) no-treatment control. Following treatment, the tooth segments were rinsed in distilled/deionized water and exposed to synthetic saliva for 24 hours. In vitro caries were formed and longitudinal sections obtained. The tooth segments were treated again and rinsed as above, before undergoing in vitro caries progression. Longitudinal sections were evaluated for lesion depth (ANOVA, Duncan multiple range analysis). Results: Following lesion formation, mean lesion depths were 173 ± 19 µm for no-treatment control; 97 ± 11 µm for APF gel; and 52 ± 12 µm for remineralizing fluid. After lesion progression, mean lesion depths were 236 ± 23 µm for no-treatment control; 184 ± 26 µm for APF gel; and 112 ± 17 µm for remineralizing fluid. Mean lesion depths for the remineralizing fluid and APF gel groups for the lesion formation and progression periods were significantly less than those for the control group (P .05). After lesion formation and progression, the APF group had significantly greater mean lesion depths than did the remineralizing fluid group (P .05). Conclusions: A remineralizing fluid containing calcium, phosphate, and fluoride in a carbopol base enhanced resistance against in vitro caries formation and progression when compared with APF treatment. Keywords: APF, calcifying fluids, caries, demineralization, enamel, fluoride, remineralization