Macrophage (M psi) migration ability and its susceptibility to methylprednisolone (MP) action, both in vivo and in vitro, were compared in young and adult guinea pigs. The random migration of oil-induced peritoneal M psi was tested by the capillary tube method. The observation of M psi locomotion from 4 to 44 h of incubation showed that cells obtained from young animals had a smaller migration ability than those of adults. Besides that, differences in reactivity to MP were observed between the compared groups. The application of MP in vivo led to the stimulation of young M psi migration in vitro, but inhibited adult M psi migration. The exposure of adult M psi to MP in vitro did not affect their motility, but migration of the young M psi, as previously, was stimulated. Thus, the presented findings report about the immaturity of young M psi random migration and its special reactivity to MP.