The possibility of a specific action of kinetin (KN) on plastidial biosynthetic events was considered by verifying the ability of this phytohormone to contrast the inhibitory effect exerted on plastid differentiation by the antibiotic myomycin (MM), a specific plastid protein synthesis inhibitor. The experiments were carried out on etiolated cotyledons of cucumber (C. sativus L.) incubated for 20 h in the dark followed by 24 h in the light in the presence or absence of KN (100 .mu.M), or MM (200 .mu.g/ml) or the 2 combined substances. In the presence of MM, chlorophyll synthesis and etioplast-to-chloroplast transformation were strongly inhibited, while in the presence of KN they both were markedly enhanced. When KN was applied simultaneously with MM, the inhibitory influence of the antibiotic was counteracted or completely nullified by the hormone. The fact that KN dominates the MM-inhibitory effect indicates that the action of the 2 molecules may be localized at similar cellular levels. Since MM exerts its activity by stably interacting with 70 S ribosomes of the chloroplast, it is suggested that KN could also act specifically at the same level.