The effect of antimicrobial agents on polymorphonuclear (PMN) chemotaxis was investigated. No significant effect was obtained in vitro with penicillins, cephems, macrolides, aminoglycosides or quinolones by the agarose plate method. However, chemotaxis was inhibited at 100 micrograms/ml or less of minocycline and doxycycline. The inhibitory effect on chemotaxis is considered to be due to chelation of Ca-ions by minocycline or doxycycline. The chelation reduced the concentrations of Ca-ions in the PMNs. The chemotactic index of minocycline increased when CaCl2 or A23187 were given in combination. Our findings indicate that a small number of the short lamellipodia were observed in PMNs preincubated with minocycline or doxycycline and these two agents affected the movement of PMNs morphologically.