Fluorescent dextrans are retained well by neurons for at least 30 days after labeling (at least in larval zebrafish neurons in vivo). However, one presumably would have to inject them into lymphocytes. (We retrogradely label neurons by disrupting their axons far from their cell bodies). I think these compounds are fairly "inert"-- we've used calcium green dextran and texas red dextran. While texas red dextran seems initially brighter, it bleaches like mad in our hands. Calcium green dextran handles repetitive imaging far more stably (and non-toxicly) in our prep, but is a calcium buffer and would dampen calcium transients in these cells. I haven't had much experience with latex beads-- but I suspect they might be better. All of this is available from Molecular Probes, Eugene OR. Might be worth a try, Don O'Malley Northeastern University