Анотация: The measurement of serum or plasma PAG concentrations is currently used as a specific method for pregnancy diagnosis in cattle. In this study, the correlation between five radioimmunoassay systems (RIA-497, RIA-706, RIA-780, RIA-809 and RIAЦPool) developed for measurement of PAG concentrations in ruminant species was investigated in plasma from pregnant Friesian Holstein females. Plasma PAG concentrations (ng/mL) measured by different RIA systems were significantly correlated between them (greater-or-equal, slanted 0.81; P < 0.001). PAG concentrations increased significantly from Day 21 (n = 27) to 30 (n = 37) after AI by use of all PAGЦRIA systems. From Day 30 to 80 after AI, lower PAG concentrations were observed when using the homologous system RIA-497. The addition of several proteinase inhibitors changed neither the non specific binding nor the B0 binding to the tracer. Our results suggest that all tested PAGЦRIA (RIA-497, RIA-706, RIA-780, RIA-809 and RIAЦPool) are highly correlated and can be useful to follow PAG concentrations in samples collected during the first trimester of gestation.