The properties of cell clones derived from locally growing and spontaneous metastases of 13762NF mammary adenocarcinoma change during in vitro growth. This has been termed phenotypic drift and is reproducible in independent experiments using different cryoprotected cell stocks. To determine whether phenotypic drift in 13762NF cell clones is the result of an en bloc shift in the properties of all tumor cells, or independent phenotypic divergence of tumor cells to produce a mixed cell population, local tumor-derived clone MTF7 was subcloned at low and high culture passage numbers in vitro. Each subclone was analyzed in vitro for cell morphology, growth rate, saturation density, karyotype and ploidy, and in vivo for experimental metastatic behavior. Subclones derived from low passage clone MTF7 (T11; tissue culture passage number 11) were relatively homogeneous in their growth rates (doubling times of 16·8-17·4 h) and saturation densities ( ∼ 2 × 105 cells/cm2); yet, these same subclones were heterogeneous in their in vitro cell morphologies, experimental metastatic potentials (means range from 0 to > 100 tumor nodules per lung), size distributions of lung tumor nodules, marker chromosomes and modal chromosome numbers. High passage MTF7 (T35; tissue culture passage number 35) subclones had similar growth rates and saturation densities, except for subclone 2, which had a doubling time of ∼ 26 h. Cell morphologies, experimental metastatic potentials (means range from 3 to > 600 tumor nodules per lung), size distribution of lung tumor nodules, marker chromosomes and modal chromosome numbers varied between MTF7 (T35) subclones. The results suggest that simultaneous, independent divergence of several phenotypes from a single cloned cell occurred to form a mixed cell population containing cells with independently segregated, unrelated phenotypes. Thus, the reproducibility of phenotypic drift in clonal cell populations was probably the result of tumor cell divergence and was not an en bloc shift in phenotypic properties of all cells. © 1984 Taylor & Francis Ltd.