Chester and Hagerstown silt loam soils were preincubated for 1 to 5 weeks with 5000 ppm C (1:1 sucrose and cellulose) and 50, 100, or 200 ppm N containing 5.25 percent excess15N as nitrate. Losses of labeled N, presumably by denitrification, were higher for the Hagerstown (65 to 70 percent) than for the Chester soil (10 to 20 percent). The N recovered was almost completely converted to organic forms. Extending the incubation from 1 up to 5 weeks had little effect on the incorporation of15N into organic forms or on recovery values. In subsequent incubations for periods up to 26 weeks, mineralized N from soil and15N-labeled sources was calculated in terms of availability ratios. These ratios did not vary greatly with time of preincubation or of mineralization period. Mineralization rate and potentially mineralizable15N increased with the total amount of incorporated15N in soils. The fraction of incorporated15N that was potentially mineralizable in both soils ranged from 34.6 to 53.2 percent, while the proportion of potentially mineralizable indigenous soil N ranged from 15 to 17.9 percent.