The carbylamine test is given only by primary amines (both aliphatic and aromatic) on heating with chloroform and alcoholic KOH, producing foul-smelling isocyanides. Aniline is a primary aromatic amine, so it gives the test. N,N-Dimethylaniline is tertiary, acetanilide is an amide (not free primary amine), and pyridine is a heteroaromatic base but not a primary amine. Hence only aniline responds positively.