Cells from cutaneous neurofibromas of three patients with von Recklinghausen's disease and skin fibroblasts from four healthy adults were cultured. After two passages, DNA and collagen syntheses were determined by measuring incorporation of [3H] thymidine and [3H] proline respectively, and expressed as the values per unit DNA content. These values from neurofibroma cells were increased by 54% in DNA synthesis and 60% (p < 0.05) in nondialyzable hydroxyproline synthesis, suggesting that neurofibroma cells in culture even after several passages still possess those characteristics corresponding to their pathological features in vivo. Collagen synthesized by neurofibroma cells appears to be normal as judged by intracellular degradation rates and SDS-polyacrylamide gel electrophoresis.