The 5-aryl-4-methyl-2-(4-pyridyl)-delta 2-1,3,4-oxadiazolines 3, previously synthesized along with isomer 4-aryl-1-methoxy-1-(4-pyridyl)-2,3-diaza-1,3-butadienes 2 from benzaldehyde isonicotinoylhydrazones and diazomethane, were tested for in vitro activity against both M. tuberculosis and some atypical mycobacterial strains as well as against human immunodeficiency virus (HIV-1). Some halophenyl derivatives, 3e, 3g, 3i, 3j, were found to display MIC ranges from 1 to 10 (micrograms/ml against H 37 Rv and a clinical isolate tubercular strain, whereas against M. avium (MAC) the MICs were higher than 20 micrograms/ml. When the combinations of oxadiazolines with ethambutol, acting as inhibitor of cell wall synthesis, were assayed on MAC strain a synergistic effect was demonstrated for 3g and 3h trifluoromethyl derivatives. The antimycobacterial profiles of 2 and 3 analogues are compared and discussed. As shown by compounds 2, no substantial anti-HIV in vitro activity was found in selected delta 2-oxadiazolines; a moderate cytotoxicity, however, appears to be a common property.