As part of a research directed to the synthesis of novel isoniazid derivatives with potential activity on mycobacteria and HIV virus, the acetophenone-isonicotinoylhydrazones 3 and the 4-aryl-1-methoxy-1-(4-pyridyl)- 2,3-diaza-1,3-butadienes 5, obtained by reaction between isonicotinoylhydrazones and diazomethane, have been prepared and tested for such activities. Both classes of derivatives showed interesting growth inhibitory activity on non-tubercular mycobacteria, including the emerging M. avium. Such activity appears to be linked to fluorine and/or chlorine presence on benzene rings. In contrast, none of the compounds submitted to the anti-AIDS in vitro screening, displayed any protection against HIV-1 virus-induced cytopathic effect in T4-lymphocyte cell lines.