In a mass screening campaign of colorectal cancer, general practitioners play an essential role for obtaining a high rate of participation from people. It is important to analyze the reasons for the participation and their actual experiences in the campaign. Out of 71 GPs participating in the campaign, 67 have been interviewed by a sociologist and 51 GPs (76%) considered themselves "motivated" for participating after an instruction course in small groups. Sixteen GPs were sceptical about the outcome or not highly motivated. However thirteen of them participated by moral obligation vis à vis their patients or for not being different from their colleagues. All GPs appreciated being actively involved in the organization of the screening campaign and considered that the public had been well informed. Such information covered the information leaflet, the press campaign at the beginning of the campaign and the recall letter for those who had not been consulted during the time when the test was distributed by GPs. The main constraint was the time spent to convince the patients to do the screening test and explain its importance. The additional time, 5 to 10 minutes per consultation was important to the GPs. They thought it was fair to be paid for their active participation. These data suggest that it is possible to motivate GPs for a mass screening campaign, on the condition that they are actively involved and participate in an instruction course for small groups.