From a practical point of view, a single source for the HL-60 cell line is needed, as well as standardized culture and differentiation conditions. Suggested conditions for a standardized OPKA protocol are available online (64). This site is updated periodically. Despite confusing experiences in the past, the HL-60 cell line has been extensively characterized and is readily available; its use has been described for the evaluation of pneumococcal vaccine induced anti-capsular antibodies by standardized OPKA, and it remains a good candidate cell line. Perhaps a single distribution center for the cell line may be established to help the pneumococcal research community. In the meantime, any laboratory interested in OPA must obtain the correct HL-60 cell line from a qualified source (e.g., ATCC).