The invention relates to methods of isolating white blood cells (WBCs) from a sample, e.g., whole blood, using magnetic particles that specifically bind to WBCs and a series of specific steps and conditions. The methods can include one or more of decreasing the viscosity of the sample prior to WBC isolation, agitating the sample at specified frequencies, and/or using a sample container arranged such that all of the sample is placed in close proximity (e.g., within 5, 2, 1, or 0.5 mm) to the source of the magnetic field. The new methods provide for isolation of WBC preparations with high yield, purity, and viability. The methods are designed for compatibility with automation protocols for rapid processing of multiple samples.