Enumeration of CD34+ cells by flow cytometry is the recognized standard for quantitating progenitor cells for peripheral blood progenitor cell (PBPC) transplantation. Although many clinical studies have confirmed that the time to neutrophil and platelet engraftment is inversely proportional to the number of CD34+ cells infused, the minimum number of CD34+ cells necessary to acheive rapid engraftment has not been satisfactorily determined. The lack of a standardized method for quantitation of CD34+ cells by flow cytometry (FCM) is often cited as the reason for this ambiguity. This report describes an FCM method for CD34+ cell determination that is simple, highly reproducible, comparatively inexpensive, and validated by excellent correlation with clinical engraftment. Pheresis samples are stained and fixed within 4 hours of collection. Two hundred fifty thousand events are acquired as list mode data using a forward scatter threshold. The discrete CD34+ population is enumerated using a CD34-phycoerythrin FL2 vs. side scatter plot and Paint-A-Gate Pro software. The method was validated by excellent statistical correlation with clinical engraftment. Using this method, we determined the number of CD34+ progenitor cells necessary to achieve rapid engraftment to be 2 x 10(6)/kg.