Fibroblasts in culture have been extensively used to study the effects of growth factors on growth of mammalian cells. They can be easily cultured and maintained for long periods of time. The objectives of these studies were to determine if viable fibroblasts could be obtained from skin of cattle and pigs, if serum, hormones or growth factors influenced the growth of these cells and if fibroblast growth was indicative of donor animal performance;A technique for acquiring a skin biopsy from ear tissue is detailed. Fibroblasts were observed to grow from this tissue explant and the cells responded to the presence of one or ten percent serum by increasing uptake of thymidine and leucine compared with cells in serum-free medium;The growth response of bovine fibroblasts to dexamethasone, estradiol, growth hormone, insulin or thyroxine additions in serum-free or medium containing 1% serum was examined. None of these hormones appeared to be a primary determinant of fibroblast growth, as measured by thymidine and leucine incorporation;Growth response of bovine fibroblasts to epidermal growth factor and insulin-like growth factor-I and combinations of hormones with growth factors was also studied. These peptides increased thymidine and leucine incorporation in the presence of one percent serum. Addition of hormones with growth factors did not dramatically improve growth;Growth of cattle and swine was compared with growth of their cells in culture. Samples were collected from newborn calves to aged cows and pigs from birth to five months of age. The potential for animal growth was expressed in cultured fibroblasts but the measurements of growth of the cells did not always predict live weight at a future point in time. As age of animals increased and growth rate relative to body weight decreased, outgrowth from the explant decreased, resulting in a negative linear relationship between outgrowth of cells from the explant and age.