Altered basement membrane protein biosynthesis by primary cultures of cpk/cpk mouse kidney. Previously, kidneys from three-week-old cpk/cpk C57/B16 mice were found to contain elevated mRNA levels for the basement membrane components collagen IV and laminin [1]. Here primary cultures of kidney epithelial cells derived from cpk/cpk C57/B16 mice were established and the production of these proteins in culture was studied. Primary cultures of cpk/cpk mouse kidney epithelial cells were observed to have a more polygonal, flattened morphology than cells from unaffected littermate kidneys. The rate of collagen IV and laminin biosynthesis was determined by means of [³⁵S] labelling studies followed by immunoprecipitation. Collagen IV and laminin biosynthesis are elevated by approximately twofold or more in primary cultures derived from 20-day-old cpk/cpk mice, as compared with parallel primary cultures derived from their unaffected littermates. Similarly, laminin Bl chain mRNA is elevated in primary cultures derived from 20-day-old cpk/cpk mice. In primary cultures derived from younger (day 11) mice, similar differences in the rates of both collagen and laminin biosynthesis were not observed between the two culture types. These observations are consistent with the previously reported age-dependent differences observed in laminin and in collagen IV gene expression in both cpk/cpk and wild-type mouse kidneys, and suggest that the regulation of overproduction of these proteins is due to an alteration in the kidney cells and not due to systemic factors.