To determine the seroprevalence of hepatitis C virus in the Philippines and compare it with the seroprevalence of hepatitis B virus infection, HBV and HCV markers in 594 serum samples collected from 392 blood donors, 123 medical and paramedical personnel, and 80 patients (45 liver diseases: 25 acute hepatitis, 9 liver cirrhosis, and 11 hepatocellular carcinoma; 28 hepatitis B carriers, and 7 chronic renal failure patients undergoing dialysis) in Davao, Mindanao Island, Philippines, were examined. HBsAg was determined by RPHA, anti-HBc by HI, anti-HBs by PHA, and HBsAg subtypes, HBeAg, and anti-HBe by EIA. HCV markers determined were anti-HCV (anti-C100-3) by ELISA (Ortho Diagnostic Systems), and anti-HCV core (anti-CP9 and/or anti-CP10) also by ELISA. Results showed that 9 (2.2%) blood donors were anti HCV positive; 69 (15.4%) were anti-HCV core positive Nine (2.2%) were HBsAg carriers; 240 (61.3%) were anti-HBs and/or anti-HBc positive (HBsAg carriers excluded from this group). Two of 123 medical and paramedical staff (1.6%) were anti-HCV positive; 11 (8.1%) were anti-HCV core positive; Eight (6.5%) were HBsAg carriers and 81 (65.8%) anti-HBs and/or anti-HBc positive. Five of 11 (45.4%) hepatocellular carcinoma patients were HBsAg carriers; 2 were anti-HCV core positive. Two of 9 liver cirrhosis patients were anti-HCV positive (1 to anti-HCV and the other to anti-HCV core).(ABSTRACT TRUNCATED AT 250 WORDS)