The results of single-dish observations of low- and high-J transitions of selected molecules from protoplanetary disks around two TTauri stars (LkCa15 and TWHya) and two HerbigAe stars (HD163296 and MWC480) are reported. Simple molecules such as CO, 13CO, HCO+, CN and HCN are detected. Several lines of H2CO are found toward the TTauri star LkCa15 but not in other objects. No CH3OH has been detected down to abundances of 10E-9 - 10E-8 with respect to H2. SO and CS lines have been searched for without success. Line ratios indicate that the molecular emission arises from dense 10E6 - 10E8 cm-3 and moderately warm (T ~ 20-40K) intermediate height regions of the disk atmosphere, in accordance with predictions from models of the chemistry in disks. The abundances of most species are lower than in the envelope around the solar-mass protostar IRAS 16293-2422. Freeze-out and photodissociation are likely causes of the depletion. DCO+ is detected toward TWHya, but not in other objects. The high inferred DCO+/HCO+ ratio of ~0.035 is consistent with models of the deuterium fractionation in disks which include strong depletion of CO. The inferred ionization fraction in the intermediate height regions as deduced from HCO+ is at least 10E-11 - 10E-10, comparable to that derived for the midplane from recent H2D+ observations. (abridged abstract)
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