Subsurface spills of high-molecular weight, multicomponent, dense nonaqueous-phase liquids (DNAPLs) are intractable for remediation by conventional techniques. This paper introduces the concept of biostabilization of the DNAPL source region as a means of achieving risk reduction at DNAPL-contaminated sites. Successful biostabilization depends upon the interplay among dissolution, degradability, and toxicity of various DNAPL constituents, difficult to predict a priori for the mixture. Bench-scale screening tests are proposed for identifying those DNAPLs that are amenable to biostabilization. The screening protocols compare four criteria: (1) microbial activity; (2) composition of the DNAPL residue; (3) aqueous phase contaminant concentrations; and (4) aggregate aqueous phase toxicity—across unbiotreated controls and in mixed versus unmixed biometers. The unmixed system represents slow dissolution from DNAPL pools in the quiescent subsurface. The protocols are developed and evaluated with DNAPL coal tar in the first paper of this set (Part I). Unmixed coal tar biometers, characterized by slow mass transfer and low-level microbial activity, exhibited reduced, aqueous-phase contaminant concentrations and aggregate toxicity, as well as stable DNAPL composition, consistently indicating favorable potential for in situ biostabilization.