A comprehensive microbiological quality assessment scheme for the benefit of all clinical microbiological laboratories in the United Kingdom was established in 1974. The main emphasis of the scheme has been on the supply of simulated clinical material for proficiency testing. Of 494 laboratories currently participating in the scheme, 84 are abroad and over 500 specimens have been distributed between 1974 and 1980. A wide variety of specimens are issued. These include specimens for: general bacteriology including isolation, sensitivity testing and serology; mycobacterial bacteriology; syphilis serology; virus isolation; general viral serology; rubella serology; hepatitis B antigen detection; electron microscopy; mycology; parasitology; antibiotic assay; public health specimens including milk and water. Laboratories are requested to examine the specimens using their routine procedures and report their results to the Microbiological Quality Control Laboratory (MQCL). The reports are analysed at MQCL and the summarised results of each distribution are sent to all participants. Each participant receives details of his individual performance on current specimens and an analysis of the previous 6 months, cumulative performance. The performance of all laboratories is reviewed twice yearly and laboratories with results significantly worse than those of their peers are offered the opportunity to seek advice and help from a National Advisory Panel of their professional colleagues. The Scheme is confidential and its main role is educational.