After resection of femurs of guinea pigs and cutting both epiphyses, bone marrow cells were flushed out with 2% fetal calf serum. A cell suspension of these bone marrow cells adjusted to 106 cells/ml was prepared by repeated pipetting and cultured in alpha medium-added 20% of horse serum at 37°C containing 5% CO2. The bone marrow culture contained multinucleated giant cells (showing positive acid phosphatase reaction) not observed in the original medium, that could be classified into 2 morphologically different types. One was discoid and lobulated cell form (with 2 to 10 small, round nuclei per cell) with undulating ruffling activity of the periphery formed by cytoplasmic fusion. The other form (with 2 to 6 large, oval nuclei per cell) was considered to arise from nuclear division of fibroblast-like cells.
