The origin of sporadic campylobacter infections has been investigated by means of a collaborative study. From a total of 1152 cases reported in North West England in 1982, Campylobacter strains isolated during one month in each quarter of the year were biotyped and serotyped. For comparison, 875 strains of Campylobacter isolated from environmental and animal sources were similarly examined. Most strains from human beings were Campylobacter jejuni; about half of them were of three serotypes. Those serotypes commonly found in human infections were frequent among strains isolated from environmental and animal sources.