To study human monocyte functions, we attempted to immortalize human monocytes by producing somatic cell hybrids between such monocytes and the mouse myeloma cell line NSI. In this study we report the successful establishment of eight hybrid cell lines that have been grown in culture for more than a year, and some of them retained part of the human chromosome complement, as well as monocyte markers and activities. Karyotype analysis of these hybrid lines revealed that cells of seven out of eight of the lines contained one to 16 human chromosomes and in four of them, more than nine human chromosomes were observed. Several of the cell lines expressed monocytic markers and functions. Thus, in two of the hybrid lines nonspecific esterase could be demonstrated in 10 to 29% of the cells, and Fc receptors were demonstrated in three of the hybrid cell lines. Significant levels of human ferritin were detected in one of the lines, and two other cell lines secreted interleukin 1-like substance into the culture medium. These results encourage us to use human-mouse somatic cell hybridization as an approach for the establishment of human monocyte cell lines, which will preserve their functions and produce monocyte-derived factors.