The light curve of the Type Ia supernova SN 1981B (in NGC 4536) is imp
ortant because Sandage et al. plan to measure the distance to the host
galaxy by means of Cepheid variables and will thus derive the Hubble
constant. Important parameters in this derivation include the peak mag
nitude as well as the decline rate of the supernova. In this Letter, I
report on the results of my quantitative light-curve template fitting
to all available data. I find observed peak magnitudes of 11.74 +/- 0
.06, 12.04 +/- 0.04, and 11.98 +/- 0.04 in the U, B, and V bands. The
infrared brightness parameter, H-20, is 12.94 +/- 0.02. For E(B - V) o
f 0.04 +/- 0.07, the dereddened magnitudes are 11.54 +/- 0.35, 11.88 /- 0.29, 11.86 +/- 0.22, and 12.92 +/- 0.04. The decline rate of SN 19
81B (Delta m(15) = 1.07 +/- 0.09) is closely matched with the average
light curve of Type Ia events, so that both the standard candle and de
cline rate calibrations will yield similar absolute magnitudes. These
magnitudes can then be used to derive the Hubble constant as soon as t
he distance modulus to NGC 4536 (mu) is measured. The combined results
from the U, B, V, and H bands are that the Hubble constant will equal
50 km s(-1) Mpc(-1) 10(0.2[(31.67) (+/-) (0.17)-mu]). The case of SN
1981B is particularly clean, so that there are not any grounds for cha
llenging the resulting H-o.