Molecular species of phosphatidylcholines were analyzed in hepatic and gallbladder bile obtained from six subjects with adenomyoma of the gallbladder (gallstone-free controls) and 27 gallstone patients undergoing cholecystectomy. Seven of the gallstone patients had been treated with cholic acid and seven with chenodeoxycholic acid for at least 8 weeks before operation. The two predominant species were 1-palmitoyl-2-oleoyl- and 1-palmitoyl-2-linoleoyl-sn-glycerophosphocholines which together accounted for 75-80% of the total amount of phosphatidylcholines. Minor species were 1-palmitoyl-2-palmitoleoyl-, 1-stearoyl-2-linoleoyl-, 1-oleoyl-2-linoleoyl-, and 1-palmitoyl-2-arachidonoyl-sn-glycerophosphocholines. Gallstone patients had a higher portion of the 1-palmitoyl-2-oleoyl species and a concomitant lower proportion of the 1-palmitoyl-2-linoleoyl species than gallstone-free subjects. The ratio between the two species was about 0.7 and 0.4, respectively, in the hepatic bile of the two groups of patients. Treatment with bile acids was associated with a normalization of the pattern of phosphatidylcholines.