Extremely low-background γ-ray spectrometry is applied to the determination of blank levels of nuclides (7Be, 60Co, 134Cs, 137Cs, 210Pb, 226Ra, 228Ra and234Th) in various chemical reagents and carriers, including Ba and Pb chemicals, BaSO4from natural barite, contrast media for medical use, and Pb in an old roof tile. Barite shows less contamination with226Ra compared with the Ba chemicals (6 - 90 mBq/g-Ba in Ba reagents, 0.7 -1.5 mBq/g-Ba in barite) . 210Pb contamination is notable in all Pb reagents, but not in the roof tile. In CsCl reagent, 134Cs, 137Cs and234Th (238U) are found to be negligible. Using the least-contaminated carrier reagents, the coprecipitation method is successfully employed to the simultaneous measurement of low-level activities of these nuclides in seawater sample.