The objective of this study was to develop a rapid, calfside test to determine serum IgG concentration using caprylic acid (CA) fractionation followed by refractometry of the IgG-rich supernatant and compare the accuracy of this method with results obtained using refractometry of whole serum. Serum samples (n = 200) were obtained from 1 d old calves, frozen and shipped to the laboratory. Samples were allowed to sit at room temperature to thaw. Fractionation with CA was conducted by adding 1 ml of serum to a tube containing CA and 0.5, 1 or 1.5 mL 0.06 M acetic acid. The tube was shaken and allowed to react for 1 min and centrifuged for 0, 10 or 20 min. Refractive index of the fractionated supernatant (nDf) was determined using a digital refractometer. Whole, non-fractionated, serum was analyzed for IgG by radial immunodiffusion (RID) and refractive index (nDw). The nDf and nDw were compared to serum IgG concentration. Mean serum IgG concentration was 19.0 mg/ml (SD = 9.7) with a range of 3.5 to 47.0 mg/ml. Serum nDw was positively correlated with IgG concentration (r = 0.86, n = 185). Fractionated samples treated with 1 ml 0.6 M AcO and 60 µl CA and not centrifuged prior to analysis resulted in a strong relationship between nDf and IgG (r = 0.80, n = 45). These results suggest that refractometry of whole calf serum provides a strong estimate of IgG concentration that can be used to determine if adequate passive transfer has occurred in 1 d old calves.