6 mice with inherited cataracts and 1 new allele of microphthalmia were recovered from 923 progeny of untreated, outbred, PT stock females that had been mated to inbred C3H/HeH strain males, whose spermatogonia had been exposed to 250 mg/kg of ethylnitrosourea (ENU). The cataract phenotypes were quite variable in expression and 5/6 showed a similar range of phenotypes. 2 of the 6 mutant mice were daughters of the same ENU-treated C3H/HeH male and probably represent repeats of the same mutation. One mutation, designated lens opacity-4 (Lop-4), has been genetically mapped to the distal region of chromosome 2. The yield of 5 presumably independent cataract mutations from 923 F1 offspring is a little higher than that reported by others in similar but larger scale experiments. Approximately 3-5% of the F1 mice examined had cataracts, yet only 6/49 (12%) of these, in the experimental group, were inherited as simple Mendelian traits. We consider that this high frequency of false positives (88%), and the incomplete penetrance and variable expressivity of the cataract mutations that were found, pose serious problems that could undermine the objective nature of the dominant cataract mutation test. We suggest that further studies be made to evaluate whether the use of inbred strains would reduce the variability in the system and so make the test more objective. However, it seems likely that the high false positive rate will continue to be a serious drawback to this test system.