Micromass cultures (MMC) of rat embryonic limb bud (LB) and midbrain (CNS) cells were applied to compare the developmental toxicity of three quinolone antimicrobials: norfloxacin (Nor), enrofloxacin (Enr) and ciprofloxacin (Cip). Cultures were exposed for 5 days to seven concentrations of drugs. Cytotoxicity was assessed by quantifying neutral red uptake; differentiation-by quantifying alcian blue uptake (LB) or by image analysis of Gill's haematoxylin stained foci (CNS). Both, LB and CNS cultures showed dose-dependent reduction in total cell number and differentiation. To distinguish specific effect on differentiation, IC(50) for proliferation (P) and differentiation (D) were calculated and P/D ratios were compared. In LB cultures all three drugs were cytotoxic (P/D ratios were 1). In CNS cultures P/D ratios were 1 (up to 2.7 for Nor, up to 4.4 for Enr and up to 16 for Cip) what can suggest specific action on differentiation. Ciprofloxacin was the most toxic and CNS cells were more sensitive than LB. The ranges of IC(50)-D values (microg/ml) were as follows: Nor (79-14), Enr (127-179), Cip (91-101) in LB cultures; Nor (22-52), Enr (38-91), Cip (3-17) in CNS cultures. With one exception (Cip in CNS culture) all drugs were classified as weak embryotoxic.