Monocyte preparations were obtained from the peripheral blood by adherence to plastic dishes. The number of monocytes obtained was significantly greater than the number of B cells recovered from similar samples. Monocytes could be frozen and thawed with relatively little loss in cell numbers or viability. Cytotoxicity tests were performed most reliably at 20 degrees C. Under these conditions, autoantibodies were rare, and normal sera were consistently negative. Serologic reactions obtained with monocytes correlated well with the known specificities of the monocyte panel. Typing results in both normal and diseased populations were similar to results obtained with B lymphocytes. Moreover monocyte cytotoxicity tests were more convenient and easier to read than similar tests performed with B cells.