Surface filters, consisting of asymmetric pore channels that passed directly from one side of the filter to the other, were made into tubular crossflow microfilters of 14 mm internal diameter. For some tests the filter tubes contained tightly fitting helices of pitches : 14, 42 and 70 mm. Thus rotational flow was induced and deposition of oil drops suspended in water was minimised. The filters provided total oil rejections of up to 97%. For all the tests the operating permeate flux was standardised at 200 l m(-2) h(-1), and transmembrane pressure was adjusted in order to achieve this. However, in the case of filtration with the helical inserts the pressures remained constant at the initial values. Hence these filtrations were run under conditions of both constant flux and pressure. When filtering with the more open side facing the challenge dispersion, the transmembrane pressure was approximately 50% of that given when filtering in the opposite configuration. The transmembrane pressures were low, values ranging from 3 to 9 kPa for the filtrations employing helices.