Fibroblasts from a beige mouse (C57BL/6J; bgJ bgJ) have been established and maintained in culture for more than 3 yr. At early passages, the mutant cells were distinguishable from C57BL/6J control mouse fibroblasts at the ultrastructural level by the presence of enlarged cytoplasmic granules. After continuous passaging, this distinguishing feature was lost from the mutant cells, correlated with their increased growth rate. Clustered, perinuclear distribution of lysosomes was retained, however, and was quantitatively different at any passage number of the beige cell line from the dispersed distribution of these organelles in control mouse fibroblasts, as analyzed by computer-aided, video-enhanced light microscopy. In somatic cell hybrids between the established beige cell line and a control human diploid fibroblast cell strain, seven uncorrected hybrid lines retained a lysosomal dispersion pattern statistically indistinguishable from that of the beige mouse cell lines. Three corrected hybrid lines had lysosomal dispersion patterns that were significantly different from the beige parent line and indistinguishable from that of the control mouse fibroblast line. Thus, lysosomal dispersion can be used objectively and quantitatively to distinguish mutant beige and control mouse fibroblasts and corrected vs. uncorrected cell hybrids made from the beige/control human somatic cell crosses.