Using data from the Federal Trade Commission's Line of Business Program and survey measures of technological opportunity and appropriability conditions, this paper finds that overall firm size has a very small, statistically insignificant effect on business unit R & D intensity when either fixed industry effects or measured industry characteristics are taken into account. Business unit size has no effect on the R & D intensity of business units that perform R & D, but it affects the probability of conducting R & D. Business unit and firm size jointly explain less than one per cent of the variance in R & D intensity; industry effects explain nearly half the variance.