One hundred and thirty-seven cultured human tumor cell lines derived from Caucasian patients were surveyed for ten of their polymorphic enzyme phenotypes. The gene frequencies in this cell line population were similar to those of normal Caucasian populations, although consistent differences in phenotype frequencies were detected at each of the loci. All 10 loci showed fewer heterozygous phenotypes and a correspondingly greater number of the common and rare homozygous phenotypes than occur in normal Caucasian populations. On the average, only 85% of the loci expressed in fully differentiated diploid cells were expressed in the neoplastic cell lines adapted to in vitro growth. There was no significant difference in the proportion of loci expressed in cells that had been passaged less than 10 times and in cells passaged more than 50 times. Consequently, it appears that there is a loss of expression in genetic marker loci from at least six different chromosomes. This loss occurs either in the in vivo tumor or in the very early stages of the cultivation of neoplastic cells derived from solid human tumors. Once the cells have become adapted to growth in vitro, the patterns of expression in their polymorphic loci remain stable for many passages.