A probability-summation model is introduced to provide quantitative cr
iteria for discriminating independent from interactive effects of mult
iple laser exposures on biological tissue. Data that differ statistica
lly from predictions of the probability-summation model indicate the a
ction of sensitizing (synergistic/positive) or desensitizing (hardenin
g/negative) biophysical interactions. Interactions are indicated when
response probabilities vary with changes in the spatial or temporal se
paration of exposures. In the absence of interactions, probability-sum
mation parsimoniously accounts for ''cumulative'' effects. Data analyz
ed using the probability-summation model show instances of both sensit
ization and desensitization of retinal tissue by laser exposures. Othe
r results are shown to be consistent with probability-summation. The r
elevance of the probability-summation model to previous laser-bioeffec
ts studies, models, and safety standards is discussed and an appeal is
made for improved empirical estimates of response probabilities for s
ingle exposures.