Placental isozyme of alkaline phosphatase (orthophosphoric monoester hydrolase, EC 3.1.3.1.) has been studied in very few Indian populations with respect to its polymorphic form. The present study surveys the alleles encountered in some Punjabi groups of Northern India. The distinguishing physico-chemical properties of this isozyme, viz. heat stability, steady increase in maternal plasma during the course of pregnancy, and unique electrophoretic mobility have been used for its typing. The Punjabi groups surveyed are Jat (Sikh), Baniya and Khatri (all endogamous castes) and Lower Castes and others (two mixed caste groups). These groups were found to be in Hardy-Weinberg-equilibrium, and some of them revealed intergroup heterogeneity, when compared with each other. Further comparison with other Indian populations, for which data were available, also revealed interesting results.