Haploid cells afford an opportunity to test some of the assumptions from bacterial genetics which have been adopted by somatic cell geneticists. Haploid cultured cell lines derived from the grass frog Rana pipiens were compared to diploid cell lines in order to test a model which predicts that recessive mutations will be expressed in diploid cells with a frequency equal to the square of that in haploid cells. Haploid and diploid monolayer cultures were compared for (1) survival after exposure to compounds known to be mutagenic for bacteria (a measure of the frequency with which lethal mutations are expressed), and (2) the induction of drug-resistant variants (putative mutants) by such compounds. The proportion of cells which survived from diploid cultures was no more than ten times that from haploid cultures, a much smaller difference than predicted. Furthermore, the frequency of drug-resistant variants was independent of polidy. Therefore, the validity of the following assumptions is in question: (1) Haploid eukaryotic cells express mutations with a frequency comparable to that in bacteria; (2) molecules which increase the frequency of stable phenotypic variants in culture do so by inducing gene mutations; (3) stable phenotypic variants whose frequency in culture is increased by putative mutagens arise as the result of gene mutation.