Novel cyanine dyes are provided which comprise first and second five- to six-membered nitrogen containing heterocyclic nuclei joined by a methine linkage; the first of said nuclei being a 2arylimino (or alkylimino)-4-aryl (or alkyl)-3-thiazoline nucleus joined at the five-carbon atom thereof to said linkage; and said second nucleus being joined at a carbon thereof to said linkage, to complete said cyanine dye.