Synopsis Sun protection factors were determined by both an in vitro method which used resin casts taken from replicas of human skin and by an in vivo SPF method. Thirty-eight product development samples were tested for the level of sun protection using both methods and the results were compared. The values obtained showed a positive relationship which was closely approximated by a log-linear regression of in vivo data on in vitro data (regression coefficient, r(2)= 0.86). It is concluded that the cast technique is quick, convenient, inexpensive and in its present form useful for screening sunscreen products prior to in vivo SPF testing.