The bioavailability of marketed nitrofurantoin capsules was compared to capsules filled with Gantrez-gelatin nitrofurantoin microcapsules of a core:coat ratio of 1:2. The collective results obtained from the analysis of urine samples of five volunteers indicated that nitrofurantoin microcapsules provided a prolonged release compared with that of the control formulation. Analysis of variance showed statistically significant differences between the control formulation and the capsule filled with the microcapsule. Non-significant intersubject variation was noticed.