Human monocytic cell fractions obtained by counterflow elutriation centrifugation (with regard to specific monocyte/macrophage characteristics: 82-88% were positive for nonspecific esterase; 86-94% for CD14) were cultured (overnight, 37 degrees C) under nonadhering conditions (polypropylene tubes). Thirty to 40% of the cells were found to differentiate into large, monocytoid cells with a dendritic morphology. These cells expressed a marker of active dendritic cells RFD1 in 76-89% and were also positive for class II MHC antigens as identified by OKIa (95-97%). An exposure of the blood monocytic cells to metrizamide (30 min, 14.5%) prior to the overnight culture enhanced this differentiation, and 47-58% of cells with a dendritic morphology were found (of which 80-87% RFD1 positive, and 95-97% Class II MHC positive). The cultured cell populations containing the cells with the morphology and marker pattern typifying dendritic cells, appeared functionally more active as accessory populations when compared to freshly isolated blood monocytic cell fractions; the cultured cells had an enhanced stimulator capability in MLR, and cluster formation with lymphocytes was more prominent. At the same time, the cultured cell population showed a decreased bactericidal activity when compared to the freshly isolated monocytic populations, and in addition, all the cultured monocytoid cells had lost non-specific-esterase activity, while only approximately 10% of cells were still positive for the CD14 marker. When U937 cells were exposed to metrizamide (14.5% concentration, 30 min) and cultured under nonadhering conditions for 36 hours, similar changes were observed (30-45% became dendritic, 20% RFD1 positive).(ABSTRACT TRUNCATED AT 250 WORDS)