Macrophage CSF (M-CSF) induces proliferation of monocyte/macrophage progenitor cells and can also activate some functions of mature cells. Three different human M-CSF cDNA (4.0, 3 to 3.5, and 1.5 kb) which are the result of alternative splicing of the single M-CSF gene have been cloned. Each of these cDNA encode a biologically active M-CSF. M-CSF transcripts are expressed in normal fibroblasts and other mesenchymal cells, and also in some hematopoietic cells such as monocytes. Normal human cells examined to date express only the 4.0-kb transcript. In contrast, a 3.5-kb M-CSF transcript was continuously expressed in two multiple myeloma cell lines (RPMI 8226 and U266/AF10) and in a bone marrow specimen of a patient with multiple myeloma. The myeloma cell lines secreted biologically active M-CSF. Resting and activated normal B lymphocytes and other B cell neoplasms examined did not express the 3.5-kb transcript, but could be induced to express the 4.0-kb transcript and to secrete M-CSF. Myeloma cells appear to be unique among hematopoietic cells in their expression of the 3.5-kb M-CSF transcript.