A solid-phase reagent for determination of urinary specific gravity (relative density) is described. This reagent strip, similar to others in the "N-Multistix" series (Ames), contains a polyacid whose acidity is sensitive to the ionic concentration in the urine in which it is immersed. As the acidity of the polyacid changes, pH changes are detected by a pH indicator within the reagent strip. In comparison studies, 84.4% of relative densities as measured with these reagent strips were within 0.005 of the corresponding results with a total-solids meter, and 89.9% were within 0.005 of the corresponding urinometer results. Adding a correction of +0.005 to the reagent-strip results for urines with high pH increased the percentage of results within 0.005 of the comparison method to 90.7% (TS meter) and 92.9% (urinometer). Lot-to-lot variability and reader-to-reader variability were both low. Reagent strip results are not affected by glucose, may be increased by albumin, and correlate with urea concentrations.