This paper uses annual mean counting rate data from detectors on two long-lived spacecraft, Pioneer 11 and IMP 8, to study the temporal and heliocentric radial distance variations of the intensity of interplanetary protons (Ep≈1 MeV) over solar activity cycle 21. The Pioneer 11 data cover the time period April 1973 through 1986 and the heliocentric radial distance range 1.0-&agr; with &agr;=2.0 (¿0.2), irrespective of solar activity as measured by the annual mean sunspot number S. Also it is found (a) that the annual mean intensity at 1 AU is approximately proportional to S, except for anomalously low values in 1979 and 1980 and (b) that the product of the annual mean intensity at Pioneer 11 by r2 is also approximately proportional to S, except for anomalously low values in 1979, 1980 (in particular), and 1981. The common ''1980 anomaly'' is attributed to gross changes in the interplanetary conditions associated with the reversal of the polarity of the sun's polar magnetic field.