Collagen gel invasion by purified tonsillar B cells in the presence or absence of rTNF or rIL-4. Results are expressed as micrometers travelled by the two faster cells from the top of the gels. Nil = no cytokine; rTNF = 1 ng/mL; rIL-4 = 100 U/mL. Incubation time = 10 hours. Results are means ± 1 SE from four experiments. *P < .05, nonparametric Mann-Whitney U-test.