Dimethyl methylphosphonate (DMMP) and exo-tetrahydrodi- (cyclcpentadiene) (JP-10), two compounds of interest to the United States Air Force, were evaluated in a battery of five in vitro short-term assays to assess their potential biological activity. The assays conducted include the Ames Salmonella/mammalian microsomal mutagenicity assay, the CHO/HGPRT gene mutation assay, the CHO/sister chromatid exchange assay, the CHO/chromosome aberrations assay, and the BALB/c-3T3 neoplastic transformation assay. In the absence of an exogenous metabolic activation system, DMMP exhibited a low clastogenic activity in the CHO/chromosome aberrations assay and was negative in the remaining four assays. Compound JP-10 also had a marginal clastogenic effect in the CHO/ chromosome aberrations assay and produced a negative (or inconclusive) response in the other assays conducted. In the presence of an S-9 fraction, DMMP produced no mutagenic response in the Ames Salmonella mutagenicity assays. Results on JP- 10 were inconclusive.