Haptotactic effect of IL-8, but not PF-4, on HMC-1 cells. The migration of cells was tested on precoated filters exposed to medium alone or to various concentrations of IL-8 or PF-4. The results are expressed as the number of countable cells per 3HPF (mean ± SEM) (n = 3). *P < .05, **P < .01.