The relative responses of fetal rat limb bones and neonatal mouse calvaria to parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) were examined in organ culture. Limb bones were cultured in dishes in BGJ + 1 mg/ml bovine serum albumin (bSA) or DMEM + 15% heat-inactivated horse serum (hS). Calvaria were either cultured in dishes with one of the above two media or in roller tubes in DMEM + hS. The order of sensitivity to PTH was: calvaria in roller tubes greater than limb bones in dishes in DMEM + hS greater than limb bones in dishes in BGJ + bSA greater than or equal to calvaria in dishes in DMEM + hS greater than calvaria in dishes in BGJ + bSA. The most sensitive system (i.e., calvaria in roller tubes) showed significant resorption in response to 10(-10)M PTH at 48 h. The least sensitive system did not respond to 3 X 10(-8)M PTH after the same length of time. The greater response in DMEM + hS compared with BGJ + bSA appeared to be due to the protein component of the medium. The order of sensitivity to 1,25(OH)2D3 was: calvaria in roller tubes = long bones in culture dishes in BGJ + bSA greater than long bones in culture dishes in DMEM + hS greater than calvaria in dishes in DMEM + hS greater than calvaria in dishes in BGJ + bSA. The most sensitive systems showed resorption in response to 10(-11)M 1,25(OH)2D3 by 72 h. The least sensitive system failed to respond to 10(-9)M 1,25(OH)2D3 after the same length of time. The nature of the protein constituent did not seem to influence the response of the limb bones to 1,25(OH)2D3. The results indicated that although the responses of the two bone systems to the calcemic hormones were qualitatively similar, media and culture conditions could markedly affect the sensitivity.