Cells procollagen alpha 1(I) transcripts were demonstrated at the interface of natural coral mineral implanted in the femur of rats after 7, 14, 21 and 28 days by in situ hybridization with digoxigenin-labelled RNA probes. Procollagen alpha 1(I) transcripts were detectable in osteoblasts between 7 and 28 days after implantation. As early as 7 days after implantation, procollagen alpha 1(I) RNA expression was also demonstrated in fibroblasts; the transcripts steady state levels decreased until the 28th day, when they subsided completely. The development of bone was significantly during the 4 weeks of implantation. Inhibition of bone development by the coralline material could not be recorded.